Prepare the marinade: Combine all the marinade ingredients in a small saucepan. Bring to a boil, stirring to combine. Remove to a food processor or blender and process until smooth. Pour into a bowl and let cool completely (you can pop the marinade into the fridge to speed up the process, but be sure it is completely cooled before adding to the meat, for food safety reasons).
Remove the silverskin and any visible fat from the pork tenderloin. Butterfly the pork, but slicing from end to end, almost, but not all the way through. Open up and press flat. Place into a plastic bag. Pour cooled marinade into bag and rub to coat the meat. Seal bag and refrigerate at least one hour and up to 24 hours. (The longer you can marinate, the better the flavour will be).
Before cooking meat, prepare salsa by combining all ingredients in a bowl. Stir to combine, cover and set aside.
*Regardless of how you are cooking, the pineapple and onion will not take as long to cook as the pork, so simply remove when done and continue cooking the pork until cooked through.
To cook on BBQ: Grill pork, pineapple rings and onion quarter until pork is cooked through and pineapple and onion are nicely grilled.
To cook in grill pan on stove top: Cook pork, pineapple rings and onion quarter over medium-high heat until cooked through and/or grilled.
To cook in the oven: Place pork, pineapple rings and onion quarter on a foil-lined baking sheet under the oven broiler, about 6-8 inches from the heat. Cook until cooked through.
Remove pork, pineapple rings and onion to a cutting board. Allow pork to rest a few minutes, then, using a sharp knife, cut the pork, pineapple and onion into small slices and toss to combine.
To serve: Heat or grill tortillas. Spoon in some of the pork/pineapple/onion mixture. Top with salsa. Serve with lime wedges, for drizzling.
Notes
If you don't have Achiote paste, you can make a homemade substitute. In a small bowl, mix together 4 1/2 tsp paprika, 1 Tbsp white vinegar, 3/4 tsp dried oregano, 1 tsp garlic powder, 1/4 tsp cumin, 1/4 tsp vegetable oil into a paste. Measure out what you need to use immediately or transfer to an airtight container and refrigerate up to 2 weeks.