Mexican Stuffed Shells

1 lb. lean ground beef
1/2 cup chopped onion
1 garlic clove, minced
1 teaspoon chili powder
1/4 teaspoon ground cumin
1/4 teaspoon dried oregano
1/4 teaspoon salt
1/2 cup mild or spicy salsa
1/4 cup light sour cream
1 1/2 ounces coarsely shredded cheddar cheese
3 ounces jumbo pasta shells, cooked according to directions and drained (about 12)
1 cup shredded lettuce
1 tomato, diced

Preheat oven to 350F. Spray an 11x7" baking pan with nonstick cooking spray. In a large nonstick skillet, over medium-high heat, cook beef, onion, and garlic, stirring frequently to break up beef, until beef is brown and onion is translucent, about 5 minutes. Drain well. Add chili powder, cumin, oregano, and salt; stir to combine. Cook 1 minute. Stir in salsa; cook, stirring frequently, 5 minutes. Remove from heat; stir in sour cream. Let cool slightly. Stuff shells evenly with meat mixture, mounding slightly. Arrange shells in prepared pan; cover with foil. Bake 15 minutes; uncover and sprinkle evenly with cheese. Bake, uncovered, just until cheese melts, 1-2 minutes. Top shells evenly with lettuce and tomato.