For mornay sauce: Place butter into a small sauce pan and melt over medium heat. Sprinkle flour over butter and whisk together. Continue to whisk for 2 to 3 minutes, to cook out the raw flour taste. While whisking add the milk until fully incorporated, ensuring no lumps have formed. Using a wooden spoon stir in the cheese until well blended followed be the remaining ingredients. Season with salt and pepper and cook for 1 minute. Remove from heat and set aside until ready to use.
Place sourdough slices onto a baking sheet and top each with 4 ounces of turkey.
Top each sandwich with a generous ladle of mornay sauce and top with a sprinkle of bacon (you can serve now if you wish). Place the baking sheet in the oven and broil each sandwich for 1-2 minutes. Top with freshly cracked black pepper and serve.