Preheat oven to 175 degrees C. Line a baking tray with aluminium foil.
Combine mayonnaise, prawn, Cheddar cheese, Parmesan cheese and garlic salt in a large bowl. Spread mixture over each muffin half. Arrange muffins on the baking tray.
Bake until brown and bubbly, about 20 minutes.
Cook's Note:
To make ahead of time, cover and freeze shrimp toast until well chilled, about 30 minutes. Remove and cut shrimp toasts into quarters and place in a plastic bag. Return to freezer until ready to use.