FOR THE BECHAMEL SAUCE:
In a saucepan on medium heat, melt the butter then add the flour, stir vigorously for 5 minutes, add the salt, pepper, and nutmeg and the milk little by little and keep stirring
After stirring for 3 minutes more, turn off the heat and reserve
FOR THE SANDWICH:
Add the bread to a baking plate, add the béchamel sauce, the tomatoes, onions, olives, oregano, cheese, and bologna, repeat the process until the bread is all in the tray, finish with the remainder béchamel sauce and garnish with tomato slices.
Put it in the oven on 350F (185C) for 25 minutes. Serve it immediately.