This may be made using sliced turkey or chicken in place of roast beef, I prefer the beef better --- leftover roast beef, thinly sliced steak or deli roast beef will work for this, if you don't have a toaster oven to toast the sliced buns you can just toast them under the broiler firstly.
Set oven to broiler heat (set oven rack to second-highest position).
Spread the softened butter on each slice of bread, then sprinkle liberally with garlic powder and a little salt.
Place in a toaster oven until toasted.
After the bread has been toasted spread liberally with mayonnaise.
Divide the roast beef over each bun then spread mustard over the beef.
Top beef with onion slices, then with tomato slices, sprinkle the tomato with salt and black pepper.
Top each sandwich with slices of mozzeralla cheese.
Place under the broil for about 3 minutes or until the is bubbly (watch closely these brown quickly!).
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