Slice the French baguettes diagonally into 3/4 inch slices.
In a medium skillet over medium heat, melt the butter. Combine the onions and garlic in the skillet. Cook and stir until tender. Set aside to cool.
In a mixing bowl, combine the mozzarella cheese, Parmesan cheese and mayonnaise.
On a cookie sheet, arrange the French bread slices in a single layer. Spread the onion and garlic mixture on the bread slices. Spread the cheese and mayonnaise mixture over the onion and garlic mixture on the bread slices. Broil about 5 minutes, until the cheese is bubbly and slightly browned. Serve immediately.
