Combine garlic powder, onion powder, dried thyme, paprika, salt and pepper in a small bowl. Rub over both sides of pork chops.
Heat the butter and olive oil in a large skillet over medium-high heat until the butter is melted. Add the pork chops to the pan and brown on both sides, about 2 minutes per side. Remove pork chops to a clean plate.
Add the onion and mushrooms to the skillet. Cook, stirring often, until they begin to soften, about 5 minutes. Add the garlic and cook, stirring, for 30 seconds.
Stir in the flour followed by the chicken broth. Cook, stirring and scraping up any bits stuck to the bottom of the skillet, for a few minutes until the broth simmers and thickens. It should lightly coat the back of a spatula. Remove pan from the heat.
Pour the contents of the skillet into the slow cooker. Place the pork chops on top. Cover and cook on low setting for 1 ½ to 3 hours. Pork chops are done when they reach an internal temperature of at least 145° F. (In my slow cooker, 1-inch thick boneless pork chops need about 1 ½-2 hours to cook through and bone-in take about 2 ½-3 hours.) Try to avoid over-cooking the pork chops, as that will make them dry.*
Optional: To further thicken the gravy, remove the cooked pork chops to a clean plate and cover with foil. Turn the slow cooker up to high. In a small bowl, stir together 1 tablespoon cornstarch and 1 tablespoon water. Pour the cornstarch mixture into the slow cooker and stir. Cook on high for 15-20 minutes, until the gravy thickens slightly. (Alternatively, you can heat the gravy in a pot on the stove over medium heat, stirring, for a few minutes until thickened.)
Season with additional salt and pepper, to taste. Serve pork chops with mushroom gravy.