Combine the red wine vinegar with 1/2 cup cold water in a small bowl. Stir in the salt, sugar and hot sauce. Add the sliced onion and let sit for at least 1 hour. These can be stored, refrigerated in an airtight container, for 1 week.
Preheat a grill pan over medium-high heat. Place tomatillos and jalapeno on the pan and allow to char on all sides, flipping with tongs as needed. Allow to cool slightly. Remove stems and add to a blender. Add half of the lime juice and half the cilantro. Pulse until combined and smooth (use caution when blending hot liquids; it's okay to have some lumps). Set aside to cool.
Add cabbages, carrot and 1/4 cup pickled onions to a medium bowl. Stir to combine. Drizzle olive oil and some of the pickling liquid into the bowl until everything is lightly coated. Set aside to marinate until ready to serve.
Clean grill pan if necessary, then heat it over medium heat and brush with olive oil. Sprinkle fillets with salt and smoked paprika. Grill fillets on both sides until cooked through and flaky, about 3 minutes per side. Warm tortillas on grill pan until soft and pliable.
Top each tortilla with 1/2 fillet of fish, tomatillo salsa and slaw. Garnish with sliced avocado, Cotija, remaining cilantro and lime wedges. Serve!
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.