In a skillet over medium heat, add ground beef and diced onion. Cook until beef is browned. Drain excess fat if needed.
Stir in taco seasoning, tomato paste, garlic powder, water, salt, and pepper. Simmer for 2–3 minutes until thickened.
Lay a flour tortilla flat. Spread nacho or queso cheese in the center. Add half of the beef mixture, then place a tostada shell on top.
Top the tostada with shredded lettuce, diced tomatoes, shredded cheese, and Taco Bell sauce. Fold the edges of the tortilla toward the center, overlapping to form a hexagon shape.
Heat a skillet over medium heat. Place the folded side down and cook for 2–3 minutes until golden and sealed. Flip and cook the other side for 1–2 minutes.
Slice in half and enjoy warm with extra sauce for dipping!