Trim all the fat from the beef shanks, cut them into pieces, and rinse them.

In a large pot, heat the oil and add as many beef shank pieces as will fit in a single layer.

Sear the meat until browned on all sides.

Remove the meat and drain off as much fat as possible. Arrange the browned meat in the slow cooker (which should now be hot).

Continue browning the remaining meat in the leftover fat, then add it to the slow cooker.

These are the ingredients you'll use to make the birria sauce.

Rinse the chiles and cook them in 2 cups of water (seeds included).

Add all the sauce ingredients to a blender, including the cooked chiles with their water. Blend until smooth.

Strain the sauce. Return any solids left in the strainer to the blender, add the remaining 2 cups of water, blend again, and strain once more.

Pour this sauce over the meat.

Cover tightly with aluminum foil. Cook for about 4 hours. Halfway through, uncover, stir, and cover again.

After about 4 hours, the meat should be fully cooked.

Serve hot, topped with chopped onion and tomato salsa. (For the salsa: cook 5 tomatoes in 3 cups of water, then blend with 2 cloves garlic, 1/2 teaspoon salt, and 1/2 teaspoon oregano.)