Place pizza crusts on a baking sheet, and brush the tops with olive oil. Scatter onion slices over crusts, spreading to evenly cover. Top with tuna, prosciutto and feta cheese, then cover ingredients with shredded mozzarella cheese and a sprinkle of red pepper flakes.
Bake pizzas for 10 minutes, or until cheeses start to brown.
Cut each pizza into 6 slices.