Preheat the oven to 380 degrees F (193 degrees C). Place pastry shells on a baking sheet.

Bake in the preheated oven until golden brown, about 20 minutes.

While pastry shells are baking, melt 1 tablespoon butter in a medium-sized skillet over medium-high heat. Add scallops and saute quickly on both sides until browned, 3 to 5 minutes; do not overcook. Remove from skillet and set aside.

Reduce heat to medium and melt remaining butter in the skillet. Add mushrooms and shallots and cook until just tender, 3 to 5 minutes.

Whisk half-and-half and flour together in a bowl and add to the skillet with the mushroom mixture, whisking until smooth and thickened, about 5 minutes. Whisk wine, sherry, lemon juice, and mustard into the cream sauce. Stir in Parmesan cheese, tarragon, and black pepper. Add cooked scallops to the sauce in the skillet, stir, and reheat briefly, 1 to 3 minutes.

Remove pastry shells from oven and cut off tops. Fill with cheesy scallops.