Butter the frying pan. Place the wine, scallops, shallots, mushrooms, and herbs in the pan, season with salt, pepper, and add the lemon juice.
Cut a piece of parchment paper with a diameter slightly larger than the pan. Butter one side of the parchment paper and press the buttered side onto the surface of the ingredients in the pan.
Place the pan over high heat. When the liquid starts to boil, remove the pan from the heat and let it rest for 15 minutes.
Remove the cooked scallops from the pan and set them aside.
Return the pan to the heat and reduce the liquid over high heat for 2 to 3 minutes.
Add the bechamel sauce and season.
Return the scallops to the pan and pour into buttered shells already filled with mashed potatoes all around.
Sprinkle the cheese over the shells and put in the oven to brown.
Serve hot.