Preheat oven to 425 degrees F (220 degrees C). Grease a sheet of aluminum foil.
Arrange mushrooms onto a sheet of aluminum foil with the hollow side facing upward. Spoon 1/2 teaspoon garlic flakes into each mushroom cap. Top each garlic portion with 1 teaspoon tomato paste; season with salt. Lie half a cheese slice atop each mushroom cap.
Bake in preheated oven until the cheese is slightly browned, about 10 minutes.