Prepare a grill and light the fire.
With a sharp knife, make 2 diagonal slits on the skin side of each fillet. Season the fish with the salt and jalapeno sauce. Put the fish in a shallow bowl and cover with the onions. Cover and refrigerate for 30 minutes.
Remove from the refrigerator and drizzle with the oil on the skinless side of the fillets. Place the fillets, skinless side down, on the hot grill. Cook for 5 to 6 minutes, then turn the fillets over and cook for 5 to 6 minutes more. Place the onions in wire basket and grill alongside the fish, turning several times to prevent burning.
Transfer the fish to a warm platter, garnish with the onions, and serve.
Yield: 4 servings