Pat the pork loin dry with a paper towel. Make cuts on the fat side.
Heat the oil in a pan. Combine the salt, paprika, garlic powder, onion powder, thyme, chili powder and pepper and season all sides of the pork well with the spice blend. Place in the hot oil and brown all sides well.
Place the browned pork loin in the slow cooker, fat side up. Brush the top and sides with the yellow mustard. Add the sliced onion and sage.
Combine the chicken broth, vinegar, brown sugar, soy sauce, mustard, and garlic together. Pour over the roast.
Cover with lid and set on low for 8 hours or high for 6 hours. Baste the meat with the sauce occasionally. Check the meat with a thermometer after 5 hours. Continue cooking and checking the meat until it reaches 145°. If using pork butt, cook to 195°F as it needs a higher temperature for the connective tissue to break down.
When the roast is done, remove and keep warm while it rests. To make the gravy, stir the water and cornstarch together and whisk into the juices in the crockpot. Set on high and let the sauce cook for another 15 minutes. Slice the pork and return it to the slow cooker or a serving platter, and spoon the sauce over the meat.