In a small bowl combine minced onion, chili powder, cornstarch, garlic powder, cumin, oregano, and pepper cayenne. Set aside. In a large skillet over medium high heat brown ground beef. Drain off excess fat and stir in taco seasoning mixture and a ½ cup of water. Simmer for 10 minutes or the liquid is absorbed and the mixture is thick. Set aside. In a small saucepan heat refried beans over low heat.
To make tostada shells- Pre-heat the oven to 375 degrees. Spray tortillas on both sides with cooking spray and place onto a baking sheet. Place into the oven and bake turning once for 10 minutes or until crispy and light brown.
To assemble tostadas- spread a thin layer of refried beans on each tostada shell. Top with taco meat, cheese, lettuce, and tomato.