Mix tuna, mayo, garlic and onion powders, and ranch dressing in a bowl, crushing tuna as much as possible.
Toast the bread. While the bread is toasting, microwave the tuna mixture for 1 minute or until hot.
Place toast pieces on a microwave-safe plate and place 1 slice of cheese on each. Spoon tuna mixture equally onto both slices. When finished, place a slice of cheese on top of both sandwiches and microwave for 30 seconds or until melted.