Step 1: Preheat your oven broiler. Ensure your broiler rack is positioned about 6 inches from the heat source.
Step 2: Toast the bread slices lightly in a toaster or under the broiler for 1-2 minutes per side until just golden. Arrange two slices of toasted bread on each of four individual broiler-safe plates or in a large shallow baking dish.
Step 3: To make the Mornay sauce, melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook, stirring constantly, for 1-2 minutes to create a light roux.
Step 4: Gradually whisk in the warm milk, a little at a time, until the sauce is smooth and lump-free. Continue to whisk as the sauce thickens, about 5-7 minutes, until it coats the back of a spoon.
Step 5: Reduce the heat to low. Stir in the Gruyere cheese, 1/4 cup of Parmesan cheese, nutmeg, salt, and white pepper. Continue stirring until all the cheese is melted and the sauce is smooth and creamy. Keep warm.
Step 6: Layer the sliced turkey generously over the toasted bread slices on each plate/dish.
Step 7: Pour a liberal amount of the warm Mornay sauce over the turkey and bread, ensuring it’s well-covered. You want a good, thick blanket of sauce.
Step 8: Carefully place the plates/dish under the preheated broiler. Broil for 3-5 minutes, or until the sauce is bubbly, hot, and golden brown on top. Watch carefully to prevent burning.
Step 9: Remove from the broiler. Immediately garnish each Hot Brown with two slices of crispy bacon, arranged in an ‘X’ pattern, and two slices of fresh tomato. Sprinkle with a little extra Parmesan cheese and paprika if desired.
Step 10: Serve hot and enjoy your authentic Classic Kentucky Hot Brown Sandwich!