Place the sliced apples and onions at the bottom of the slow cooker.
In a small bowl, whisk together apple cider, maple syrup, dijon mustard, salt, and pepper.
Place the pork tenderloin on top of the apples and onions and pour the sauce over the top.
Cover and cook on low for 4-6 hours, or until pork reaches an internal temperature of 145°F.
Remove pork from the slow cooker and let rest for 5 minutes.
In a small bowl, mix the cornstarch and water until smooth. Add to the cooking liquid in the slow cooker.
Turn to high and cook for 5-10 minutes, or until the sauce thickens.
Slice pork, serve over apples and onions, and spoon maple gravy on top. Garnish with thyme if desired.