Trim crusts from bread slices, and discard. Cut 6 bread slices in half diagonally. Place all bread on a baking sheet; broil 3 inches from heat (with electric oven door partially open) until golden, turning once.
Arrange 1 whole and 2 half bread slices in a single layer in 6 lightly greased individual oval baking dishes.
Melt 2 tablespoons butter in a large skillet over medium heat; add mushrooms, and sauté 5 minutes. Drain and remove from skillet.
Melt 1/2 cup butter in skillet over low heat; whisk in flour. Cook, whisking constantly, 1 minute. Gradually whisk in milk; add onion, salt, and pepper. Cook over medium heat, whisking constantly, until thickened and bubbly. Add Cheddar cheese, stirring until melted.
Pour half of sauce evenly over toast; top evenly with sliced ham, sliced turkey, mushrooms, and remaining sauce.
Broil 5 inches from heat (with electric oven door partially open) until bubbly and lightly browned; remove from oven.
Top each with 2 bacon slices,and sprinkle evenly with Parmesan cheese. Serve immediately.