Peel and trim off the ends of the onion. Halve and slice into thin half-moons. Spread these out on the bottom of the slow cooker insert, tossing with a 3/4 teaspoon fine sea salt.
Trim any excess fat from the pork (if it has an especially thick fat cap, you can trim it down to a thinner layer). Slice the pork lengthwise into 1/2" wide steaks. Season with salt on both sides and lay on top of the onions.
In a blender, add 1/4 cup pineapple juice (from the can of pineapple rings), the apple cider vinegar, chili powder, garlic powder, oregano, cumin, salt, chipotle powder, cloves and pepper. Blend until smooth.
Pour the sauce over the pork and onions. This might not seem like enough liquid but the pork and onions will also release liquid as it cooks.
Cover and set on low for 6-7 hours.
Turn on the broiler.
Remove the pork from the slow cooker and lay flat on a baking sheet. Broil for 3-5 minutes, until parts are darker and crispy, then flip and broil for another 3-5 minutes.
Slice the pork crosswise into chunks. Chop the pineapple rings into small chunks. Set out your preferred tortillas, lettuce cups or veggie rice. Serve the pork, drizzling more sauce over the top, with the onions, pineapple and cilantro, if using.