Pupusas with Cheese, Beans and Curtido

Steps

1. Start with the curtido, which needs to pickle for at least 1 hour. Slice the cabbage into 1/8-inch (3 mm) shreds, grate the carrot and cut 1 white onion into thin half-moons. Put everything in a colander set in the sink and slowly pour 1 liter (about 4 cups) of boiling water over it, tossing with a fork. Let drain for 2 minutes, then press the vegetables firmly with the back of a spoon to squeeze out the water.
2. In a large bowl, whisk the cider vinegar, 1/2 teaspoon of the salt, the sugar and 1 teaspoon of the oregano rubbed between your palms. Add 1 jalapeño sliced into thin rings (seed it if you're wary of the heat), then the drained vegetables. Toss well, pack down, cover and refrigerate for at least 1 hour, ideally overnight; this resting time isn't counted in the active times.
3. Make the salsa roja. Heat 1 tablespoon of oil in a saucepan over medium heat. Add 1/2 white onion in large chunks, 2 whole garlic cloves, the second jalapeño, seeded and halved, then the tomatoes cut into quarters. Cook for 10 minutes, stirring now and then, until the tomatoes collapse and their skins wrinkle. Blend until smooth with 1 teaspoon of oregano, return to the pan and simmer for 5 minutes to thicken slightly. Season with 1/4 teaspoon of salt and set aside.
4. Make the beans. Finely chop the remaining 1/2 onion. In a skillet, heat 2 tablespoons of oil over medium heat and cook the onion until soft, 4 minutes. Add the last garlic clove, minced, and the cumin, stir for 30 seconds, then add the drained black beans and 50 ml (3 1/2 tablespoons) of warm water. Mash them right in the skillet with a potato masher and cook for 6 to 8 minutes, scraping the bottom: the mash should turn into a thick paste that holds its shape on the spoon. Season with 1/4 teaspoon of salt and let cool completely.
5. Shred the mozzarella (a firm, low-moisture pizza mozzarella shreds far better than a fresh ball in brine) and crumble the queso fresco. Mix the 2 cheeses into the cooled beans to make a dense filling. Divide it into 8 portions of about 70 g (2 1/2 oz).
6. Make the dough. In a large bowl, mix the masa harina with 1 teaspoon of salt. Pour in 550 ml (2 1/3 cups) of warm water a little at a time, mixing by hand, then knead for 2 minutes. The dough is ready when it feels like modeling clay: smooth and supple, it doesn't stick to your fingers, and a ball pressed between your palms doesn't crack at the edges. If it cracks, add water a spoonful at a time; if it sticks, add a little masa. Cover with a damp towel and let rest for 5 minutes.
7. Divide the dough into 8 balls of about 115 g (4 oz) and keep them under the towel. Add 1 tablespoon of oil to a bowl of water and dip your hands in it. Flatten one ball in your palm into a 4-inch (10 cm) cup, set a portion of filling in the center, bring the edges up and pinch them closed, then roll it back into a ball. Pat it from hand to hand, turning as you go, into a disk 5 inches (12 cm) wide and 1/2 inch (1 cm) thick. If a crack shows the filling, patch it with a bit of dough.
8. Heat a cast iron skillet or griddle, dry, over medium-high heat for 5 minutes. Add 4 pupusas without crowding and cook for 4 to 5 minutes without touching them, until the bottoms release on their own and are covered in brown spots. Flip and cook for another 4 to 5 minutes: the cheese bubbles out of the seam and toasts into a lacy crust on the iron. Keep them warm wrapped in a towel while you cook the other 4.
9. Drain the curtido. Serve 2 hot pupusas per person with a big handful of curtido and the salsa roja on the side to spoon over. Eat them with your hands or a fork, tearing the pupusa to dip it in the salsa.