cooked beef trotter (about 400 grams) • apple cider vinegar (about 80 ml) • thinly sliced onion, divided (about 80 grams) • refried beans (about 240 grams) • lettuce, cleaned, shredded, and rinsed • dried oregano, crushed by hand • thick sour cream (about 120 ml) • finely shredded queso fresco (about 120 grams) • Pickled jalapeños, to taste • tostada shells • water (about 80 ml) • avocado