Heat oil in a 12 nonstick skillet over medium-high heat. Season fish with salt and pepper and add to skillet, skin side down. Cook until skin is browned and crisp, about 4 minutes. Remove from skillet; set aside. Add onions to pan; season with salt and pepper. Cook, stirring, until soft, about 4 minutes. Add tomatoes, cover with lid, and cook until soft and releasing their juices, about 8 minutes. Add fish, skin side up, and cook until done, 3 to 4 minutes. Sprinkle with parsley before serving.