The sauce can be prepared several hours in advance and refrigerated; bring to a simmer before proceeding.
Combine in a bowl and mix well:
Melt in a medium skillet over medium heat:
Add and cook, stirring, until softened but not browned, about 2 minutes:
Add and cook, stirring occasionally, until the mushrooms are softened, about 7 minutes:
Add:
Increase the heat and simmer until the wine has almost evaporated, about 3 minutes. Add:
Bring to a gentle boil and boil until thickened, about 5 minutes. Preheat the broiler. Reduce the sauce to a simmer and add:
Cook until the scallops are no longer translucent, about 1 ½ minutes. Remove from the heat and stir in:
Spoon the mixture into scallop shells or individual gratin dishes. Sprinkle the bread crumb mixture over the scallops and sauce. Broil until golden brown on top and bubbling around the edges, about 1 ½ minutes.