Spread margarine on 1 side of each slice of bread.
Cut each slice diagonally into 4 triangles.
Place 8 triangles with buttered sides upright against sides of ungreased square baking dish, 8x8x2 inches.
Arrange 8 triangles, buttered sides down, on bottom of dish.
Mix cheeses, onion, garlic, mustard and salt; spread over bread.
Arrange remaining 8 triangles, buttered sides up, on cheese mixture.
Mix remaining ingredients; pour over bread.
Cook uncovered in 325Â° oven until knife inserted in center comes out clean, about 1 hour 10 minutes.
Let stand about 10 minutes before serving.