Heat oven to 375°F.
Char 1 poblano pepper directly over a gas flame or under the broiler, turning until the skin is blistered and blackened on all sides.
Transfer the pepper to a bowl, cover, and let it steam for 5-10 minutes; remove the stem and seeds, peel away the skin by scrapping the back of a knife across the pepper, and dice.
Heat 1 tablespoon olive oil in a large cast-iron skillet over medium heat. Add the diced white onion and cook, stirring occasionally, until softened and lightly caramelized, about 8-10 minutes.
Stir in 2 minced garlic cloves, the chopped poblano, and season with kosher salt, then cook for 1 minute more.
Top with 8 ounces shredded Oaxaca cheese, 8 ounces shredded Monterey Jack cheese, and 4 ounces shredded pepper jack cheese, spreading them into an even layer.
Transfer to the oven and bake until golden and bubbly, about 5-6 minutes.
Remove the skillet from the oven, and clear away anything flammable. Pour 1/4 cup tequila or mezcal carefully over the molten cheese, and ignite it with a long-handled lighter while standing back.
Let the flame burn out completely before moving or serving the skillet. Garnish with chopped cilantro.
Serve immediately with warm corn tortillas, warm flour tortillas, or tortilla chips while the cheese is still hot, stretchy, and gooey
