Add vegetable oil to a large skillet, heat to medium-high. Season the pork loin roast on all sides with garlic powder, salt and pepper. Sear pork for 2-3 minutes per side or until golden brown.
Place the pork loin in the slow cooker, fat side up. Cover with sliced onion if using.
Deglaze the skillet. Add water, brown sugar, molasses, balsamic vinegar, soy sauce and minced garlic to the skillet. Over medium heat scrape and stir all the browned bits from the pan into the sauce, simmering for 3-4 minutes.
Pour the sauce over the pork, cover and cook on low for 4-5 hours. Or on high for 2-3 hours.
When done cooking, remove the meat from the pot and transfer it to a cutting board. Cover with foil and let rest 5-10 minutes before slicing or shredding.