Reconstitute the chiles by adding them to a saucepan with 3 cups of water and bringing them to a full boil. Once the mixture comes to a full boil, turn off the heat and allow the chiles to sit in the hot water for at least 10 minutes while you prepare the rest of the sauce.

Add the chopped onion, garlic, dried oregano, cumin seed, achiote paste, orange juice, white vinegar, pure maple syrup, and sea salt to a high-powered blender.

Remove the stems and seeds from the chiles and place them in the blender along with 2 cups of the liquid used to boil them. Blend on high until the sauce is smooth.

Transfer the chopped pork to a large bowl or sealable glass container.

Pour the marinade over the pork and stir everything around (this is easiest to do with your hands, but be sure to wear gloves!) until the pork is well-coated in marinade. Cover the bowl with plastic wrap (or seal the airtight container with a lid) and refrigerate for 12 to 36 hours. Note: you can absolutely marinate the meat for less time, but for best results, marinate for 1 full day or longer. 

When you’re ready to cook the al pastor, plug in your slow cooker and transfer the marinated pork (with the marinade) to your slow cooker. Secure the lid and cook on low for 8 hours or high for 4 to 5 hours.

Strain the meat from the juices and transfer it to a large baking sheet in a single layer. Broil in the oven on the High broil setting for 5 to 10 minutes, or until crispy.

As an alternative, you can crisp up the meat in a large cast iron skillet. Just be sure to heat it in a single layer for optimal crisp.

Serve al pastor as tacos, burritos, or with rice and beans or your choice of sides. You can spoon the juices from the crock pot over the pastor meat for added flavor.