Load the slow cooker. Place the pork chops in the bottom of a large slow cooker. Add the sliced mushrooms and onion over and around the pork chops.
Make the sauce. In a medium bowl, whisk together 1½ cup of the beef broth, minced garlic, Worcestershire sauce, whole-grain mustard, mushroom powder, garlic powder, onion powder, paprika, kosher salt, and black pepper. Pour evenly over the pork chops.
Cook. Cover and cook on LOW for 4–6 hours or HIGH for 3–4 hours, until the pork chops are very tender and easily pull apart with a fork. For the best texture, I recommend cooking on LOW.
Remove the pork chops and thicken the gravy. Carefully transfer the pork chops to a plate and cover loosely to keep warm. Whisk the cornstarch with the remaining ½ cup cold beef broth until completely smooth. Whisk the slurry into the liquid in the slow cooker. Once the gravy has thickened, stir in the heavy cream.
Finish. Taste and adjust the salt and pepper as needed. Return the pork chops to the slow cooker and spoon the mushroom gravy over the top. Serve warm.