Brown the beef in a large nonstick skillet over high heat, breaking it into smaller pieces as it cooks. When no longer pink add 2 teaspoons of the cumin along with the remaining dry spices, onion, bell pepper and garlic, stir and cook 2 to 3 minutes until soft, then transfer to the slow cooker along with 1/2 cup water, bay leaf and tomato sauce. Cover and cook on high 3 hours or low 6 to 8. Discard the bay leaf and add the remaining teaspoon of cumin.
Heat the taco shells according to the directions, then assemble placing 1/4 cup beef in each shell, topped with lettuce, 1 tablespoon cheese and tomato.