Thinly slice the red onion and add it to a large jar. Top it with the hot water, vinegar, sugar and salt and shake well. Refrigerate and marinate for at least 20-30 minutes before serving. Keep refrigerated for up to 2 weeks.
Spiced Mushrooms
Toss the sliced mushrooms with the smoked paprika, chili powder, cumin, salt and a drizzle of olive oil, about 2 tbsp. Add in the diced white onion and chopped cilantro. Massage together with your hands and let marinate for 10 minutes.
To cook, heat a large skillet on medium heat and add in another tbsp of olive oil. Add the mushrooms and cook on medium/high heat for 10-15 minutes, tossing every couple of minutes.
Fire-roasted Corn
Wash and trim the corn on the cob and pat dry. Turn your stove on to a high flame and place the corn directly onto the fire. Cook each cob for about 8 minutes total, flipping once you start to hear popping after a couple of minutes. If you don't have a gas stove, you can prepare the corn on a grill-pan or under the broiler.
Once ready, sprinkle with flakey salt and slice the corn off the cob using a sharp knife.
Assemble the tacos
Heat the tortillas on both sides over a medium/high flame to char, or on a skillet with no oil on medium heat.
Layer with the cooked mushrooms, sliced avocado, corn, pickled onions, cilantro and hot sauce. Top with a fresh squeeze of lime juice.