Stuffed Bell Peppers

5 green, red or yellow bell peppers, halved
1/2 onion, chopped
1 tablespoon butter
1 pound ground beef
1 can cream of mushroom soup
1 can Ro-Tel diced tomatoes with green chiles
3 cups cooked long grain rice
Salt and pepper to taste
Garlic powder to taste
4 thick slices Velveeta cheese

Boil one of the bell peppers, then dice it. Brown the bell pepper with the onion in the butter. Saut