Preheat your oven to 375°F (190°C).
Boil the lobsters for about 8 minutes, then remove and let cool.
Remove the meat from the shells, chop it into bite-sized pieces, and set aside. Keep the shells for later.
In a saucepan, melt the butter, add the flour, and cook for 2 minutes to make a roux.
Gradually add the white wine and cream, stirring constantly until the sauce thickens.
Add the mustard, salt, and pepper. Stir in the lobster meat and half of the Gruyère cheese.
Fill the lobster shells with the mixture, place them in a baking dish, and sprinkle the remaining cheese on top.
Bake for 15-20 minutes, or until the top is golden and bubbly.