Make the curtido: rinse shredded cabbage in a mesh strainer and pour boiling water over it, then rinse with cold water; drain and dry well.
Toss the cabbage with shredded carrots, chopped pickled jalapeño, 2 tablespoons jalapeño brine, 2 tablespoons white vinegar, 2 teaspoons olive oil, 1/4 teaspoon dried oregano, and 1/2 teaspoon kosher salt until evenly combined; refrigerate until ready to serve.
Make the dough: in a large bowl whisk together 2 cups masa harina and 1 teaspoon kosher salt; add 2 cups warm water and 1 teaspoon vegetable oil and mix with a spatula until a dough forms; cover with a damp towel while you prepare the filling.
Prepare the bean filling: heat 1 teaspoon olive oil in a medium skillet over medium heat; add minced poblano, minced onion, chopped cilantro, and minced garlic and cook until softened, about 2–3 minutes.
Add 3/4 cup canned black beans (with liquid) and 3/4 teaspoon kosher salt; cook, stirring, until most liquid evaporates and beans thicken, about 2–3 minutes; transfer to a bowl and let cool 5 minutes, then stir in 3/4 cup shredded mozzarella.
Form the pupusa dough: knead the dough in the bowl briefly, then turn onto a lightly dusted surface and form into a slightly flattened ball; divide into 8 equal pieces and keep covered with a damp towel.
Prepare a bowl of 3/4 cup water mixed with 1/2 teaspoon oil to keep your hands damp while shaping; working one at a time, roll each piece into a ball and flatten into a 1/4-inch disk.
Scoop about 3 level tablespoons of the bean-and-cheese filling into the center of each disk; bring the edges together to seal the filling, then gently flatten into a 5–5½ inch disc without squeezing out the filling.
Heat a large skillet or griddle over medium-low and lightly spray or oil the surface; cook the pupusas 7–10 minutes total, flipping halfway, until browned and crisp on both sides.
Keep cooked pupusas warm on a sheet pan in a 200°F oven while you cook the rest, then serve topped with sliced avocado and the prepared curtido.
