Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut fish into four serving-size pieces, if necessary. Measure thicknes of fish; set aside. Fill a large skillet with water to a depth of 1 inch. Bring water to boiling; reduce heat.
Meanwhile, arrange onion slices in the bottom of a steamer basket. Place fish on top of onions. Sprinkle fish with garlic salt and pepper. Place steamer over simmering water. Place celery on fish. Cover and simmer gently until fish begins to flake when tested with a fork. Allow 4 to 6 minutes per 1/2-inch thickness of fish. Discard vegetables. Serve fish with salsa or flavored butter.