Step 1) Combine onions, jalapeno peppers, lime juice, oil and smokey mesquite seasoning in a resealable bag; add pork. Refrigerate for up to two hours.
Step 2) Combine salsa ingredients, seasoning to taste with salt and pepper. Cover and refrigerate until serving.
Step 3) Remove pork from marinade; discard marinade. Grill pork over Direct Medium heat for about 15 minutes, until internal temperature is at least 150°F. Remove to platter and allow to rest at least 10 minutes. Cut into slices and serve with Avocado Salsa.