Create tostada shells: Preheat oven to 400 degrees. First wrap 4 tortillas in a damp paper towel. Heat in microwave for 10-20 seconds to soften. Coat each tortilla on both sides with cooking spray. Turn a 12-cup muffin tin upside down. Nestle a tortilla in the space between 4 cups to form a “bowl.” Repeat with 3 more tortillas, making 4 bowls total. Bake until firm and beginning to brown, about 12-15 minutes. Transfer to a wire rack or plate to cool. Repeat with the remaining 2 tortillas.
To make turkey filling: In a large, nonstick pan over medium-high heat. add ground turkey and onions. Brown while continuing to break up turkey into a crumble. Once browned, add beans, corn, taco seasoning, and water. Stir until mixture comes to a boil. Reduce heat to low and simmer uncovered for 10 minutes.
To make each tostada: Fill each shell with about ¾ cup turkey meat. Top each with 1 cup lettuce and chopped tomatoes. Drizzle each with 1 tablespoon skinny ranch or Thousand Island dressing.