Season the pork with the salt and pepper and brown it in the oil over medium-high heat, 2 minutes per side.
Spread the onion and apples in a 5- or 6-quart slow cooker and set the tenderloins on top.
Whisk the broth, mustard, maple syrup, vinegar and thyme and pour the mixture around the pork.
Cover and cook on low until the thickest part of the pork reaches 145°F, 3 to 3 1/2 hours.
Transfer the pork and apples to a platter and rest the meat for 10 minutes.
Whisk the cornstarch with the cold water, stir it into the cooker and cook on high until the sauce thickens, 8 minutes.
Slice the pork across the grain and spoon the apple-onion sauce around it.