Preheat the broiler. Cook the bacon in a large skillet over medium heat until crisp, about 10 minutes. Transfer to a paper towel-lined plate. Pour out all but about 1 tablespoon fat from the skillet.
Make the gravy: Add the onion to the skillet and cook, stirring, until soft, about 3 minutes. Add the flour and cook, stirring, 1 more minute. Increase the heat to medium high, add the milk and chicken broth and bring to a boil, stirring. Reduce the heat to medium low and simmer, stirring, until slightly thickened, about 6 minutes. Remove from the heat and stir in 1 cup cheese. Season with salt and pepper.
Arrange the bread on a baking sheet. Spread each slice with mustard, then drizzle with some of the gravy and top with the sliced tomato. Toss the turkey with the remaining gravy in the skillet. Divide the turkey evenly among the bread slices, then sprinkle with the remaining 1/4 cup cheese. Broil until golden, about 2 minutes.
Crumble the bacon over the sandwiches; sprinkle with the parsley.