Best-Ever Pupusas

In a medium bowl, add cabbage, onion, and carrots. Pour boiling water and vinegar over. Add salt, oregano, and sugar and toss to combine. Add more boiling water as necessary to completely cover vegetables. Let cool to room temperature, then cover and refrigerate at least 1 hour.  In a large bowl, combine masa harina, salt, and warm water. Knead with your hands until a smooth dough forms. Let rest 10 minutes.  Divide dough into 12 balls. Using your thumb, make an indention in the center of each ball and fill with a tablespoon of cheese, beans, or a mixture of both. Pinch dough together to cover filling then use your hands to flatten into a 4” disk, about ¼” thick.  In a large skillet over medium heat, heat oil. Working in batches, cook pupusa until golden, 3 minutes per side.  Serve with cabbage relish.