Step 1

FOR THE BIRRIA: Combine all ingredients in Dutch oven and bring to boil over high heat. Cover; reduce heat to medium-low; and simmer until meat is tender, about 2½ hours. Off heat, let meat rest in broth for 30 minutes. Using ladle, skim fat from surface of birria broth and transfer it to shallow dish or wide-mouthed bowl; set aside.

Step 2

Using tongs or slotted spoon, transfer meat to large bowl. Using tongs or potato masher, smash meat until finely shredded; set aside.

Step 3

Using immersion blender, process birria broth mixture until smooth, about 3 minutes. (If using jar blender, fill only halfway for each batch, especially if broth is hot. Process until smooth, about 20 seconds per batch.) Strain processed broth through fine-mesh strainer set over large saucepan; discard solids.

Step 4

Stir 2 cups birria broth into shredded birria meat. Season with salt and pepper to taste and cover with aluminum foil to keep warm. Season remaining broth with salt and pepper to taste and place over low heat to keep warm.

Step 5

FOR THE TACOS: Heat oil in 12-inch nonstick skillet over medium heat until shimmering. Drag 1 side of 3 tortillas through reserved birria fat and place tortillas in skillet, fat side down (tortillas will overlap slightly). Top each tortilla evenly with ¼ cup mozzarella. Spread scant ¼ cup birria meat over half of each tortilla. Cook until most of cheese has melted, 2 to 3 minutes.

Step 6

Sprinkle red onion and cilantro to taste over each taco, then fold nonmeat half of tortilla over meat using spatula and tongs. Cook tacos until crisp on both sides, 1 to 2 minutes per side. Transfer tacos to serving dishes. Wipe skillet clean with paper towels. Repeat with remaining tortillas, fat, mozzarella, meat, red onion, and cilantro. (You needn't reapply vegetable oil to skillet.) (Extra meat can be refrigerated for up to 4 days.)

Step 7

Portion ½ cup warm birria broth into each of 4 to 6 small crocks (1 for each person) and sprinkle with cilantro to taste. (Extra broth can be refrigerated for up to 4 days.) Serve tacos with broth; dunk tacos in broth before each bite.