Preheat oven to 375º F.
In a large frying, brown hamburger. Drain grease.
Stir in pizza sauce and tomato paste. Add oregano, garlic powder, salt and pepper. Stir in water. Simmer for 15 minutes, stirring frequently, over medium to medium-low heat until very, very thick, set aside. Do not skip this step, and cook for the full amount of time.
Place bottom halves of hamburger buns onto a baking sheet pan. Pile the meat mixture onto each bun. Top each with a slice of cheese.
Melt butter and stir in the garlic powder and salt. Brush over the inside tops of the hamburger buns and place on a second sheet pan. (*If just baking a few, you can put them all on one sheet. Just watch the the tops and remove them if they get done before the cheese melts on the bottom topped ones.)
Place both baking pans into the oven, (*top buns on upper rack and meat topped buns on rack below), and bake for about 6 - 8 minutes or until the cheese is melted and the tops are lightly toasted and golden browned. *Watch the tops so that you can take them out when they're browned to your liking. Then let the second sheet go a bit longer if needed.
Place on tops and serve hot with a side salad, cole slaw, vegetable, or mac 'n cheese!