Core the apple and evenly slice it. Slice the onion into a similar shape.
Combine the water, balsamic vinegar, maple syrup, garlic, and red pepper flakes in a small bowl, and set aside. (this is your sauce)
Lay the apples and onion into the bottom of your slow cooker, and top with the pork tenderloin. Make sure the pork is touching the bottom of the dish so it will brown on the bottom.
Sprinkle with salt, pepper, and nutmeg, and top with sauce.
Cook on the low setting for 5 to 6 hours.
Remove the meat, slice, and serve with apple, onion, and sauce over top. I recommend serving it over sautéed veggies like green beans or kale!
Enjoy!