Line a 10x12 or larger rimmed baking sheet with foil or parchment paper, set aside. Preheat oven broiler to high, adjust oven rack to the lowest position.
In a saucepan over medium heat, melt the 4 Tablespoons unsalted butter. Once the butter has melted, whisk in the 4 Tablespoons all-purpose flour until no streaks remain and blended well to create a roux. Cook roux for 3-4 minutes, stirring regularly.
Slowly add 3 cups half and half to the saucepan, whisking constantly to prevent lumping. Cook mixture until bubbly and thickened, whisking regularly, 3-5 minutes.
Once the mixture has thickened, remove the saucepan from the heat. Add half of the 1 cup shredded sharp white cheddar cheese , half of the grated parmesan cheese, ¼ teaspoon salt, 1/8 teaspoon black pepper, and 1/8 teaspoon ground nutmeg, whisk until melted and thoroughly combined. Add the remaining half of the cheddar cheese, mix until melted and combined, remove from heat and set aside.
Assemble the hot brown. Lay 4 slices of thick sandwich bread in a single layer on the prepared baking sheet. Divide the 12 ounces turkey breast, thickly sliced evenly between each piece, about 3 ounces per slice. Top each with a tomato slice, or two depending on size.
Pour about 1 cup mornay sauce over each sandwich, top with the remaining grated parmesan cheese. Place two slices of bacon on each sandwich.
Place baking sheet in oven, broil for about 2 minutes or until mornay sauce is bubbly. NOTE: When using the broiler, it is important to watch it constantly. Do not walk away while it is broiling, you need to watch it the entire time so it doesn't burn.
Remove from the oven, then enjoy!
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.