Combine the marinade ingredients and pour over the fish in a shallow pan.
Allow it to sit in the fridge for about an hour, flipping every 15 minutes
Warm your grill to high heat. Sprinkle a bit of taco seasoning on both sides of the fish and set each steak on the grill directly over the fire. Cook the fish as you would a beef steak, about 12 minutes total, flipping once.
Remove the fish and allow it to cool, sitting at room temperature for about 5 minutes.
Meanwhile, make the sauce by mixing the taco seasoning and a squeeze of fresh lime juice into the sour cream. You don’t measurements — this is easy to make to taste.
Shred the fish with a fork or your hands, and fill each taco shell. Garnish with tomato, avocado, cheese, and cilantro. Add a dollop of the sauce to the top and enjoy with a margarita or a beer.