Make the adobo paste: Chop enough of the onion to make 2 tablespoons and place it in a molcajete or food processor with the garlic and serrano. Sprinkle with salt and grind to a very smooth paste. Mix in the olive oil, achiote paste, and cumin. Add 2 teaspoons each of vinegar and lime juice, then add more as needed to make a fairly thick paste.
Cut the fish into several large pieces and coat them thickly on all side with the adobo. Wrap and chill for at least 1 hour.
While the fish is marinating, prepare any of the sauces or salsa you are making homemade as well as the cilantro sprigs, lime wedges and cabbage. Cut ¼ onion from the remaining piece of white onion and slice it thinly.
Heat a grill or grill pan to medium-hot. Place the fish in a grill basket and press half the jalapeños and sliced onions onto it. Turn the fish over and repeat on the other side.
Grill the fish without moving it until it is well cooked on one side, then turn it carefully, trying to keep as much of the spice paste on the fish as possible. If the fish is very thick, you may need to finish cooking it in a 350-degree oven. Transfer the cooked fish to a warmed serving platter.
To make a taco, spoon a little avocado cilantro sauce (or guacamole) onto a tortilla. Break up the fish and put some on the sauce. Top with a squeeze of lime, a small amount of each salsa, and some shredded cabbage.