Here's a recipe for fish tacos that you won't be able to stop eating until they're all gone. There are a lot of moving parts in this recipe, but none of them are particularly difficult to accomplish, and the layers of flavor they impart to the tacos are well worth the trouble. If you split up the different parts of the recipe with a few extra hands in the kitchen, it'll be a piece of cake.
In a large bowl, mix all of the ingredients together and whip to thoroughly combine. Keep very cold in the refrigerator until ready to use. (This recipe makes a considerable amount of batter, there may be extra.)
Place the onion in a large bowl and toss with the salt and sugar. Let marinate for about 1 hour at room temperature. After 1 hour, strain off the liquid and add the vinegar, cucumber, and habanero salsa. Keep cold in the refrigerator.
In a large mixing bowl, toss all of the ingredients together until well combined. Keep cold in the refrigerator.
Preheat the oven on the lowest temperature setting.
Place the fish into a large glass baking dish and toss with the oil, garlic, and half of the lemon juice. Season with salt and pepper, to taste, and marinate for about 1-2 hours. Place the flour and the tempura batter into 2 separate bowls. Season the flour with salt and pepper.
Heat a saucepan filled 1/3 full with canola oil to 350 degrees. Dip each piece of fish first into the flour and then into the tempura batter. Be sure to shake off any excess batter and then fry the fish in batches until golden brown, crispy, and cooked through.
Keep the cooked fish warm in the oven on roasting racks until all of the fish is cooked. Wrap the tortillas in a paper towel and microwave briefly until warmed, about 1 minute. Toss the cabbage with the remaining lemon juice and season with salt and pepper, to taste.
Lay out the tortillas and top each tortilla with a piece of fried fish. Spoon the tartar sauce on the fish and then finish each taco with equal parts cabbage and the pickled onion-cucumber mixture. Serve immediately.