Place your chicken breasts in a resealable plastic bag and pound the thicker parts of the breast with a rolling pin until they match the height of the thinner parts. The goal is about 3/4 inch thick.
In a food processor or blender, puree the marinade ingredients until smooth. Transfer to a resealable bag along with the chicken breasts and refrigerate for 2-3 hours.
Clean and oil your grill grate and preheat to about medium-high (375-450 F).
Grill the pineapple rings on both sides on medium-high heat, until grill marks appear. Set aside to cool and finely dice.
Remove the chicken breasts from the marinade, scraping off some of the spicy seeds if you want to cut the heat a bit. Sprinkle generously with salt on both sides.
Add the chicken breasts to the grill and cook for 4 minutes on the direct medium-high heat until brown, flip to the other side to brown the other side. Transfer the chicken to indirect heat (an area of the grill without the flames directly on), cover with the lid and cook until the breasts are perfectly cooked through – to 165 F.
Remove from the grill, let it sit for 5 minutes, then slice into strips.
Meanwhile, mix together the pineapple, avocado, red onion, cilantro, lime juice, honey, chili, salt and pepper.
To serve, pile the chicken onto the lettuce leaves, top with the salsa and a sprinkle of feta cheese and cilantro, if desired.