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.
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.