Cook the bacon according to its package instructions, crumble and then place to the side.
Toast the bread and warm the turkey or ham.
Preheat your oven to broil and ensure it a rack is on the top section.
In a sauce pan, melt the butter and add the flour. Stir to combine and continue to cook until you eliminate the flour taste, about 3 minutes, when it smells toasted. Add the heavy cream and whisk to combine and ensure there are no lumps.
Add the black pepper, ground mustard, cayenne, and a pinch of salt.
Once it comes to a simmer, turn off the heat, and add 1/2 Cup of the cheese, melt, and stir to combine.
On a sheet pan layer the sandwich with the bread on the bottom, then a layer of turkey or ham, and sliced tomatoes. Ladle the sauce on top of the sandwich and top with the remaining cheese.
Place the sheet pan in the oven and broil the sandwich until the cheese is melted.
Remove the sandwich and place each sandwich on a plate. Top each sandwich with the cooked bacon, chopped chives, and season with salt and ground black pepper.
NOTE: You can substitute with a white or yellow cheddar on top of the sandwich before broiling.