Prepare the Red Onion Condiment
Slice onion crosswise in very thin slices. Heat vinegar, sugar and ½ teaspoon of salt in a small, heavy, non-reactive saucepan until sugar dissolves.
Remove from heat and stir in sliced onion. Let stand about one hour.
Drain the onion prior to serving.
Prepare the Fish Marinade
Mix all the marinade ingredients together to form a wet paste and smear on the fish about ½ hour before roasting.
Prepare the Crema Sauce
Whisk together the mayonnaise, crema and lime juice to make the sauce.
Finish & Serve
Grill, roast or pan fry the fish until cooked through and flaky. Roasting should take about 10 minutes at 350° F.
Serve fish in warm tortillas with sauce and other condiments.