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 loin, fat side up into the crockpot, then add the onion and sage.
Combine the chicken broth, vinegar, brown sugar, soy sauce, mustard, and garlic together. Pour over the roast.
Place the lid tightly on the pot 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°.
When the roast is done, remove and keep warm. Stir the water and cornstarch together and whisk into the sauce. Set on high and let the sauce cook for another 15 minutes. Return the meat to the pan and spoon the sauce over the meat.