Special equipment: metal or wooden skewers (soak in water before using)
Add the pineapple juice, achiote paste, chipotles, garlic and 1 1/2 teaspoons salt to a food processor and puree until uniform. Reserve about 1/2 cup of the marinade for glazing. Put the remaining marinade and the pork in a resealable plastic bag and marinate for 2 hours in the fridge, rotating if necessary halfway through.
Heat a grill to high heat; clean and oil the grill grates.
Thread the onion rounds on the skewers, drizzle with oil and season with salt and pepper. Grill the onions, developing a deep char on each side, about 10 minutes per side. Set aside to cool.
Lightly season the tenderloins once more, then grill on all 4 sides, frequently basting with the reserved marinade, until golden brown and the internal temperature registers 145 degrees F, 8 to 10 minutes per side. Transfer to a plate, tent loosely with foil and rest for 5 minutes.
Mince the cooled onions and season again. Slice the pork and serve with a dollop of Avocado Crema and garnished with chopped cilantro and charred onions.
Put the cilantro, Mexican cream, avocado and lime zest and juice in a food processor and run until smooth. Season with salt and pepper. Cover with plastic wrap pressed on top until serving, to keep the avocado from browning.
Recipe courtesy of Jeff Mauro