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The main dish, French classic

Lobster thermidor

Lobster thermidor is a classic French dish, lobster halves filled with a mustardy cognac cream sauce and topped with Gruyère, for 6 people. It takes 45 minutes of active work, can be fully assembled the night before up to the gratin step, rates as moderately difficult, and costs €14.50 per person. The key to success is stirring in the egg yolks off the heat so the sauce stays smooth.

Lobster halves gratinated in a mustardy cognac sauce, everything is ready the night before, all that's left is to pop them in the oven.

Lobster thermidor
For
6 people
Work
45 min
Total time
1 h 30
Prep
Prepared the day before
Difficulty
Medium
Cost per person
€14.50
Calories per person
560 kcalcalculated by Kiloach, your weight coach

Under the golden Gruyère, lobster meat mingles with a creamy sauce lifted by cognac and a touch of mustard, with tarragon tying it all together with its anise note. This is a dish that sets a festive table the moment it arrives, with the unmistakable look of classic French cooking and shells that make an entrance on their own.

It's the dish for a dinner where you want to make an impression without spending the evening in the kitchen, a holiday meal, a birthday, or a family gathering in fall or winter. Build the rest of the menu around it with a light starter beforehand and a dessert that isn't too heavy, so the lobster stays the star.

The schedule

What happens when, so you can be with your guests instead of in the kitchen.

  1. The day before 4 steps, 18 min
  2. The same day Nothing to do
  3. Before the guests arrive 1 step
  4. When it's time to serve 1 step, 12 min

The night before, cut up the lobsters, make the cognac and mustard sauce, and fill the shells before topping them with Gruyère and chilling them. On the day, there's nothing to do before guests arrive except take the baking sheet out of the fridge half an hour ahead. At serving time, run the lobsters under the broiler until golden, and serve right away with the mesclun.

The ingredients

6 people

Fish counter

  • Cooked lobsters, 500 g each 3

Fruits and vegetables

  • Shallots 3
  • White mushrooms 250 g
  • Tarragon 0.5 bunch
  • Mesclun greens 150 g

Dairy

  • Butter 60 g
  • Heavy cream 30 cl
  • Egg yolks 2
  • Grated Gruyère 80 g

Pantry

  • Cognac 6 cl
  • Dry white wine 15 cl
  • Dijon mustardoften in the pantry 2 tbsp
  • Saltoften in the pantry 1 pinch
  • Pepperoften in the pantry 1 pinch

Allergens Shellfish, Lactose, Mustard, Eggs

The recipe, moment by moment

The day beforeWhat's better after resting overnight in the fridge.

  1. Cut the cooked lobsters in half lengthwise, remove the meat from the tails and claws, and cut it into large chunks. Keep the shells.

    Ask your fishmonger to split them for you, it saves ten minutes of work.

  2. Saute the chopped shallots and mushrooms in butter, flambé with the cognac, add the white wine, and let it reduce by half. 10 min

    Move the pan off the heat when you flambé, the flame goes up high.

  3. Add the cream and mustard, let it thicken, then off the heat stir in the egg yolks and tarragon, and mix with the lobster meat. 8 min

    Adding the yolks off the heat keeps the sauce smooth so it doesn't split.

  4. Fill the lobster shells with the mixture, top with Gruyère, and keep them chilled on a baking sheet.

    Prop the shells up with crumpled foil so they stay level.

Before the guests arriveThe last hour, right before the doorbell rings.

  1. Take the baking sheet out of the fridge half an hour before baking.

    A dish that's lost its chill browns more evenly under the broiler.

When it's time to serveThe final touches, right in front of your guests.

  1. Broil at 220 degrees until the top is golden and bubbling, and serve with a dressed mesclun salad. 12 min

    Stay by the oven, the gratin can go from golden to burnt fast.

Our tips to get it right

  • Pick cooked lobsters with firm meat, it should feel firm to the touch, a sign it isn't already overcooked before it even goes in the oven.
  • Go easy on salt in the sauce before adding the Gruyère, the cheese and shells already bring their own saltiness.
  • Spread the filling evenly between the shell halves so they all broil at the same rate and nothing burns before the rest turns golden.

The variations

  • Swap the cognac for armagnac or brandy, the result stays close with a slightly rounder note.
  • For a more budget friendly version, use langoustines or langoustine tails instead of lobster, keeping the same sauce.
  • To save time on the day, make the sauce and filling the night before as directed, then just warm them gently before broiling, skipping the half hour rest out of the fridge.

What to serve it with

  • Dressed mesclun salad, as in the recipe
  • Rice pilaf or steamed potatoes on the side
  • Dry white wine, like Chablis or Sancerre

Storage

The filled lobster halves keep for 24 hours in the fridge, covered, before baking. This dish doesn't freeze well, the texture of the meat and sauce would suffer too much. Reheat only in the oven, never in the microwave, to keep the gratin crisp.

Questions about this recipe

Can I make this with frozen lobster?

Yes, as long as you thaw it completely and drain it well before removing the meat. The meat will release a bit more water, so let the sauce reduce a little longer before adding the egg yolks.

How do I know the lobster is properly cooked before stuffing it?

A cooked lobster has a bright red shell and firm, opaque meat, never translucent. If you buy it already cooked from the fishmonger, just check that the meat pulls away from the shell easily.

Can I use a different cheese instead of Gruyère?

Yes, Comté or Emmental both work well for the gratin. Avoid sharper cheeses that would overpower the delicate lobster flavor.

Do I need to keep the shells whole to serve?

Yes, that's what gives the dish its generous, recognizable look. Just ask your fishmonger to split the lobsters cleanly in half so the shell halves sit steady on the plate.

Can a lobster thermidor be made ahead?

Yes, it's even recommended. The recipe needs 45 min of work in total, split up like this, the day before, 4 steps, before the guests arrive, 1 step, when it's time to serve, 1 step.

How long does it take to make a lobster thermidor?

Allow 1 h 30 in total, including resting and cooking time, of which 45 min is actual work. The recipe is rated "medium".

How many calories are in a lobster thermidor?

About 560 kcal per person, for a recipe meant for 6 people. The calories are calculated by Kiloach, your kilo coach, the app that partners with Mon Chou.

How much does a lobster thermidor cost?

About €14.50 per person, that's €87.00 for 6 people, based on average supermarket prices.

What allergens does this recipe contain?

This recipe contains shellfish, lactose, mustard, eggs. Remember to ask your guests about their allergies before putting your menu together.

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