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.
In a large bowl, toss the drained cabbage with shredded carrots, chopped jarred 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; refrigerate until ready to serve.
Make the dough: in a large bowl whisk together 2 cups masa harina and 1 teaspoon kosher salt, then stir in 2 cups warm water and 1 teaspoon vegetable oil until a soft dough forms; cover with a damp towel and rest while making the filling.
Prepare the filling: heat 1 teaspoon olive oil in a medium skillet over medium heat and cook the minced poblano, minced onion, chopped cilantro, and minced garlic until softened, about 2–3 minutes.
Add 3/4 cup canned black beans (with liquid) and 3/4 teaspoon kosher salt to the skillet and cook over medium, stirring, until most liquid evaporates and the mixture thickens, about 2–3 minutes; transfer to a bowl to cool about 5 minutes, then stir in 3/4 cup shredded mozzarella.
Form pupusa dough: knead the dough in the bowl briefly, turn out onto a lightly dusted surface, and shape into a ball; if it cracks add water 1 tablespoon at a time. Divide into 8 equal pieces and keep covered with a damp towel.
Work each piece one at a time: keep a small bowl of 3/4 cup water mixed with 1/2 teaspoon oil for wetting your hands. With slightly damp hands roll a piece into a ball, flatten to a 1/4" disk, place about 3 level tablespoons of filling in the center, close the edges to encase the filling, then gently flatten to a 5–5½" disc without tearing.
Heat a griddle or large skillet over medium-low and spray or oil lightly. Cook pupusas 7–10 minutes, flipping halfway, until browned and crisp on both sides. Keep finished pupusas warm on a sheet pan in a 200°F oven while cooking the rest.
Serve warm topped with sliced avocado and curtido.