In a small sauté pan, heat 4 tbsp. unsalted butter and add the mushrooms and 1/2 of the leeks. Cook over medium low heat until the mixture forms a loose paste, about 20 minutes. Add the parsley and tarragon, and season the mixture with salt and pepper, and cook for 2 minutes longer. You’ll know it’s done when most of the moisture from the mushrooms has evaporated. Divide the mixture into the bottom of the scallop shells forming a base for the scallops to sit on.
In a small saucepan over medium heat, add the sherry, remaining leeks, bay leaf, and 2 tbsp water, and simmer until reduced by half. Strain the liquid and set aside, discarding the bay and leeks.
In a small sauté pan, add 1 tbsp. butter and flour and make a roux, cooking over medium heat for 3 minutes. Add the reduced sherry mixture and 35% cream, and whisk to combine. Cook, simmering, for about 7 minutes, or until fully thickened and the flour has been cooked out. Season with salt and pepper, and add the lemon juice. Whisk to combine and set aside.
In a medium sauté pan, add the remaining 1 tbsp. butter and olive oil, and bring to medium high heat. Season the scallops with salt and pepper, and place in the pan to sear. Cook on each side for 2 minutes and transfer the seared scallops to the prepared scallop shells, placing them on top of the mushroom mixture.
Spoon 2 tbsp of the cream mixture on top of each scallop, and top with the grated Comté cheese, dividing the cheese evenly.
Place the scallops under the broiler for 3-5 minutes, or until the cheese has melted and lightly browned, and the scallop is fully warmed through.
Serve with a salad of fresh greens and herbs, and some crusty bread.