In a large nonreactive bowl, combine the cabbage, carrots, onion, lime zest, and salt. Reserve the jalapeños for now.

Work the salt into the vegetables using your hands for about 2 minutes. If you’ve ever massaged kale for a salad, that’s the motion you want to employ here. In slightly less technical terms, it’s basically smooshing. The vegetables should begin releasing their liquid.

Use a wooden spoon to stir the jalapeños into the shredded vegetable mixture.

Use your hands to pack the curtido tightly into a quart mason jar, a handful at a time.

Once all the curtido is packed into the jar, push it down with your fist, the back of a wooden spoon, or both, a few times. Now the curtido should be just covered with its own brine.

Now, wash your hands! You don’t want jalapeño hands.

Secure the jar with an airlock and allow to ferment for up to 2 weeks. Begin tasting for doneness after 3 days.

Cover, label, and refrigerate for long-term storage

Combine masa harina and salt in a medium bowl. Add water and mix until combined, let sit covered for 5 minutes.

Squeeze some dough in your palm, dough should feel like soft play-doh, not too dry that it cracks when you squeeze it but not too wet that it squishes through your fingers and sticks to the side of the bowl. If it is too dry add more water, 1 tablespoon at a time, until it is the right consistency. Cover with plastic wrap and let rest 10-15 minutes.

Take about 1/3 cup of dough and use your palms to flatten into a 1/2-inch thick circle. Place a tablespoon or so of the black beans in the middle. Fold dough up over beans and pinch together to make a ball.

Gently flatten the ball back into a 1/2 inch patty, being careful not to rip open the dough. You can take a piece of dough from the outside of the pupusa and cover any rips or tears. Form into a circle and place on a parchment-lined sheet pan. Cover with plastic wrap to prevent from drying out. Repeat with remaining dough.

Heat a cast iron skillet or comal over medium heat for 5 minutes. Grease the pan with a teaspoon of oil, using a paper towel to coat it all over.

Rub a little oil all over the outside of a pupusa and place on the hot pan. Cook for 5 minutes on one side or until toasted and a little charred in a few places. Flip and cook on the other side. Repeat with remaining pupusas.

Serve hot with the Lime Curtido and sour cream if desired.