Finely slice onion into strips. In a baking pan spread the onions evenly, add the tomatoes. Drizzle tomatoes and onions with the oil. Season with the salt, pepper flakes and black pepper, coat the tomatoes and onions evenly. place in oven for 10 minutes, reduce heat to 350 and continue to roast for 10 more minutes. In a bowl, add the feta cheese add the tomato mixture and gently stir until combined.