In a mixing bowl, combine shredded cabbage, grated carrot, and dried Mexican oregano. Boil hot water and mix it with apple cider vinegar, kosher salt, and granulated sugar until dissolved. Pour this mixture over the cabbage and carrot, tossing to coat evenly. Let sit for at least 4 hours, ideally overnight.

In a large bowl, whisk together masa harina and a pinch of kosher salt. Gradually add cold water, stirring until a soft, pliable dough forms, about 5 minutes. Cover with a damp cloth.

Take a portion of the dough, roll it into a ball, and flatten it into a disc. Spoon a mixture of shredded mozzarella, pickled jalapeños, and roasted butternut squash into the center, fold edges to seal, and flatten gently.

Heat a cast-iron skillet over medium heat, drizzle with olive oil, cook pupusas for 4–6 minutes on each side until golden and crispy.

Transfer cooked pupusas to a plate, serve hot with curtido and lime wedges.