Preheat oven to 425º F
In a saucepan over medium heat add the butter until completely melted.
Add in the garlic and cook for about a minute.
Sprinkle in the flour while whisking constantly. Let cook for at least a minute up to a couple minutes.
Add in the milk slowly while whisking. Continue to stir until it begins to thicken. Turn off the heat when it's thickened.
Assemble the sandwiches by placing half of the bread slices in a thin layer on sheet pan
Spoon the béchamel on the slices, then a couple slices of deli "meat," top with the cheese. Place the second slice on top of each and spoon another layer of béchamel and top with cheese to finish.
Cook in oven for 12-15 minutes until beginning to bubble/brown. You can broil 2-3 more minutes to finish, but keep an eye when doing this.
Remove from oven and serve with flakey sea salt.