In a large bowl, combine the flour, milk, cream, wine, mustard, salt and pepper until smooth; set aside.
In a large skillet, saute scallops and onion in butter until scallops are opaque. Remove with a slotted spoon. Add milk mixture to the skillet. Bring to a boil; cook and stir for 2 minutes or until thickened.
Drain scallops. Add the scallops, mushrooms and tarragon to the sauce; heat through. Stir in Asiago cheese until melted.
Divide scallop mixture among four 10-oz. baking dishes. Combine the topping ingredients; sprinkle over scallop mixture. Broil 6 in. from the heat for 1-2 minutes or until golden brown.