Heat oil in a large skillet over medium heat. Saute onions until soft. Stir in garlic, and saute about 1 minute. Add tomatoes, and cook until they begin to soften. Stir in wine, olives, capers, and red pepper flakes. Heat to a simmer.
Place sea bass into sauce. Cover, and gently simmer for 10 to 12 minutes, or until fish flakes easily with a fork. Transfer fish to a serving plate, and keep warm.
Increase the heat, and add butter to sauce. Simmer until the sauce thickens. Stir in cilantro. Serve sauce over fish.