Prepare Mornay Sauce:
Melt butter in a medium saucepan over medium-high heat. Whisk in flour; cook, whisking constantly, 1 minute. Gradually whisk in milk. Bring to a boil, and cook, whisking constantly, 1 to 2 minutes or until thickened. Whisk in Parmesan cheese, salt, and pepper.
Preheat broiler with oven rack 6 inches from heat.
Place bread slices on a baking sheet and broil 1 to 2 minutes on each side or until toasted.
Arrange bread slices in a lightly greased broiler-safe baking dish. Top bread with turkey slices. Pour hot Mornay Sauce over turkey. Top with Parmesan cheese.
Broil 6 inches from heat for 3 to 4 minutes or until lightly browned and bubbly. Remove from oven.
Top sandwiches with tomatoes and bacon. Serve immediately.