Cook bacon in a skillet over medium heat for 8 to 10 minutes until crisp and drain. Pour off excess fat, add turkey and broth, cover and warm gently for 5 to 8 minutes until turkey reaches 165°F.
Meanwhile, melt butter in a small saucepan, whisk in flour for 1 minute and gradually whisk in milk. Simmer for 2 minutes until smooth, then remove from heat and stir in cheddar, Parmesan, salt and pepper.
Heat the broiler. Toast bread on both sides on a broiler-safe sheet for 1 to 2 minutes total, then arrange warmed turkey over each slice.
Add tomato slices, spoon cheese sauce over the turkey and lay 2 bacon slices across each open sandwich. Broil 6 inches from the heat for 2 to 3 minutes until sauce bubbles and browns in small patches.
Rest for 1 minute, lift with a broad spatula and serve one open-faced sandwich per person with a knife and fork.