Salt and pepper the meat side of the fish, spray with nonstick cooking spray if desired. Place skin side down on a hot grill. Cook about 30 seconds and flip over. The skin should separate from the meat. Cook 1 to 2 minutes, and turn over, to finish cooking the other side of the fillet. When cooked through, around 1 minute more, depending on the temperature of your grill, remove from heat. Serve hot with ranchero sauce.
Saute onion, red pepper and poblano chile until soft and translucent. Add the tomatoes, bring to a boil. Add seasonings. Simmer for 15 to 20 minutes. Serve hot.