Process tomatillos until smooth. Add salt, sugar and garlic and finely chopped chipotles — process until smooth. Split the sauce into two parts; one for marinate and one for salsa.
Add 1 tablespoon oil to marinade and pour over fish in a glass plate — allowing to marinate for 1 hour or more.
Add avocados to salsa. Stir vigorously until creamy; set aside out of refrigeration.
Brush fish with oil and season lightly with salt and pepper. Grill until just done. Transfer to plates and spoon over salsa. Serve with black beans, Spanish rice and Mexican beer or margaritas.
