Lamb or chicken wrapped in maguey membrane and slow-steamed with guajillo chile is pure comfort. This high protein, family dish delivers fresh, tender flavors unique to Central Mexico.
Soak the dried maguey leaves (if using) in hot water until soft; set aside, or prepare parchment if substituting.
Toast guajillo and pasilla chiles on a dry skillet over medium heat for 1 minute, pressing gently until fragrant; soak chiles in hot water for 10 minutes.
Blend softened chiles with onion, garlic, cumin, oregano, peppercorns, salt, vinegar, and oil until smooth; use 1/2 cup soaking water for a spreadable paste.
Rub the chile marinade thoroughly over lamb or chicken pieces, ensuring each is coated well. Marinate for at least 30 minutes (up to overnight) in the refrigerator.
Lay out maguey leaves or parchment; place a portion of meat and some carrots, potatoes, a bay leaf, and an epazote sprig in each. Wrap tightly and secure with kitchen twine.
Arrange the packets seam-side up in a large steamer or Dutch oven with a rack. Pour water beneath the rack, cover tightly with a lid or foil.
Steam gently on low heat for 3 to 4 hours, ensuring a simmer but never a boil; add water as needed to prevent drying out. Meat should be very tender.
Unwrap carefully. Serve the mixiotes hot, directly in their wrappers for the full aroma and experience.