Heat oven to 400 degrees F.
Sauté scallops, shallots and tarragon in 1 teaspoon butter in large nonstick skillet until scallops are translucent, 3 minutes. Remove with slotted spoon to bowl.
Add milk, wine and mustard to skillet. Bring to boiling. Stir together cornstarch and water in small cup; stir into skillet. Cook, stirring, until thickened. Remove from heat. Stir in cheese until melted. Add to scallops.
Mix together bread crumbs, Parmesan, remaining butter, salt and pepper in bowl.
Spoon scallop mixture into six 1/2-cup ovenproof ramekins or scallop shells. Top with bread crumb mixture.
Bake in 400 degree F oven for 10 to 13 minutes, until cheese is bubbly and crumbs golden. Serve with lemon. Makes 6 appetizers.