Spray a 6 or 8-quart slow cooker with cooking spray, add the sliced onions to the bottom of the cooker, and set aside.
Pat the pork tenderloins dry with a paper towel.
Season the pork tenderloins with salt and pepper all over.
In a large skillet over medium-high heat, add the olive oil. Once hot, sear one pork tenderloin at a time on all sides until browned. Place the seared tenderloin on top of the onions in the slow cooker. Repeat with the other tenderloin.
Pour the beef broth around the tenderloins.
Sprinkle the gravy mix on top evenly.
Add slices of butter on top of each tenderloin.
Cook on low for 2 - 2.5 hours or until an internal temperature reaches at least 145°F and the onions are softened, or until your desired internal temperature.
Slice and serve immediately with the juices as a gravy.
