Preheat oven to 350°F. Cook manicotti shells according to package directions; drain, cover and set aside.
Combine ground beef, onion and garlic in a heavy-bottomed skillet. Cook until browned, about 5 minutes. Add chili powder, cumin, oregano and salt. Stir in salsa. Continue cooking, mixing well, about 5 minutes.
Remove from the heat and stir in sour cream.
Stuff prepared manicotti shells with meat mixture and arrange in a 2-quart baking dish. Cover with aluminum foil and bake for 15 minutes. Uncover and sprinkle with cheddar cheese.
Bake uncovered until cheese melts, about 2 minutes. Serve with shredded lettuce, chopped tomato and chopped green bell pepper.