To make the Mornay sauce, melt the butter in a saucepan over medium heat. Add the flour and whisk constantly for 2-3 minutes until the mixture turns light brown.
Gradually add the milk to the saucepan, whisking constantly to prevent lumps from forming. Cook for 5-7 minutes, stirring frequently, until the sauce has thickened.
Remove the saucepan from the heat and stir in the grated Parmesan cheese. Season with salt and pepper to taste.
Assemble the Sandwich
Preheat the oven to 350°F.
To assemble the sandwich, place one slice of toasted bread in an au gratin or other oven proof dish. Top the bread with the slices of turkey, followed by the tomato.
Spoon the Mornay sauce over the top of the sandwich, making sure to cover everything evenly.
Place in the preheated oven and bake for 10-15 minutes, or until the Mornay sauce is bubbly and lightly browned.
Place the bacon criss cross over the top and leave in a minute longer to heat the bacon. Remove the sandwich from the oven and serve hot. Enjoy!
Notes
Note: Some versions of the Hot Brown sandwich also include a sprinkling of paprika over the top of the Mornay sauce for added flavor and color.