Place ground beef in a mixing bowl, and add onion, jalapeño, taco seasoning, garlic, cumin and cilantro flakes (optional). Mix well with hands. Make 1/4-pound patties. Place one cheese slice on top of patty, folding it if necessary so that it is directly in the center of the patty with room on all sides. Then place another patty on top of the other and squeeze the edges together, keeping the cheese inside. Grill, fry, or bake to your liking. When done, place on bun and top with lettuce and tomato.