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.