Place boneless pork shoulder pieces in even layer in a slow cooker. Add onion and water. Cover and cook on LOW for 8 hours or HIGH for 3 hours.
Remove carnitas and onion from slow cooker with slotted spoon and discard liquid. Use a fork to shred carnitas into smaller pieces. Set aside.
About 1 hour before carnitas is done: Preheat oven to 400° F. Wash potatoes, poke both sides a few times with a fork, then tightly wrap each in foil. Place on a baking sheet and roast 40-50 minutes, rotating halfway, until cooked and tender. Remove from oven, let cool slightly, then remove foil.
Cool potatoes 10-15 minutes, slice in half. Cool longer if centers are still too hot to touch. Use a spoon to scoop out center of each potato half, leaving some flesh, into a large bowl. Use a potato masher to soften. Set skins aside.
Add 1-½ cups cooked carnitas, 2 cups cheese, sour cream, beans, milk, green onion, chili powder, salt and pepper to bowl. Stir until combined. Scoop mixture into center of potato halves, creating mounds on each. Sprinkle tops with desired amount of shredded cheese.
Place on foil lined baking sheet and roast at 400° for 10 minutes, or until center is warm, and cheese is melted and golden. Cool slightly and top with cilantro and extra sour cream before serving.
Recipe will provide extra carnitas. Add more to potatoes skins if desired, or use for leftovers or another recipe.
Wrapping potatoes in foil when baking helps them cook faster and stay tender. A few signs they are done is a sizzling sound from steam inside, a nice smell coming from oven, and centers feel soft when poked with a fork.
To save time: You can microwave the whole potatoes instead of roasting. Heat on high about 10 minutes, skipping step 3 in instructions.
Potatoes will be very hot from oven. Cool slightly before removing foil, then again before cutting in half and scooping out flesh.
Be sure to leave a thin layer (about ¼-inch) of potato flesh so they hold their shape for filling.
Storage: Leftover can be stored sealed in fridge 3-5 days. Freezing not recommended as the potato and filling will get soggy when defrosted.
How to reheat leftover loaded potato skins: Simply bake in oven again 400° F for 10-12 minutes and serve.