Cut leeks (white and light green parts) in thin slices.
Cut slices of bread into circles, using the rim of a glass or a cookie cutter.
Spread mayonnaise on each round.
Place slice of leek on top.
Sprinkle grated parmesan cheese.
Preheat oven to 350° F (180° C). Arrange canapés on a baking sheet.
Bake until slightly golden.