In a skillet, cook onion in olive oil over medium high heat until tender and translucent. Stir in garlic, and cook for 1 minute. Add tomatoes, tomato sauce, capers, olives, vinegar, salt, pepper, and crushed red pepper (if using).
Reduce heat, cover, and simmer for a minimum of 30 minutes, or up to 2 hours, time permitting. Serve over pasta, fish, or chicken, and top with crumbled feta or grated Parmesan cheese. Enjoy!