In a small pot bring some water to a boil and add the chiles. Turn off the heat and let the chiles steep until they are pliable.
Take the chiles out of the hot water and remove stems and seeds. Add them to a blender and add the 1/2 cup of pineapple juice. Blend for a few minutes until very smooth and pass through a fine mesh sieve. It will be a bit thick so use a spatula or spoon and keep stirring the mixture against the sieve.
Pour this pineapple-chilies mixture back into the blender or food processor and add cumin, oregano, garlic, achiote paste, 1 tablespoon of white vinegar and 1 tablespoon of salt and blend until smooth.
In a large bowl or dish, slather thick marinade between layers of pork and add the sliced onions in between, placing each new layer of pork directly on top of each other. Cover with plastic wrap and let it marinade in the refrigerator for one hour (and no more because those pineapple enzymes are powerful!).
In a very large skillet, heat it on high heat until it is scorching hot. Sprinkle a little more salt on both sides of the meat. Sear the pork for 1-2 minutes on each side. Throw in the onions that was in your marinade as well. Your pork should have enough fat on it so you won’t need any additional oil but if it doesn't add, just a little to your pan. Sear off all of the pork and then dice them roughly into 1/2 inch cubes.
Return the diced meat into the pan in batches (don’t crowd your pan) and cook the meat until it gets nice and brown about 10 minutes.
Add all the marinating onions and some pineapple slices in your pan, grill or griddle so they become caramelized.
Heat some small tortillas on the pan or griddle.
To assemble your tacos, place two tortillas and pile a little meat on top. Add some onions and cilantro, maybe some tomatillo salsa and some grilled pineapple if you so desire.