Heat 3 Tbsp of oil in a large frying pan. Sprinkle garlic salt on the roast and sear on all sides in the frying pan. This seals in the juices and flavor of the meat and gives it a deep brown color on the edges.
Place the roast in the crock pot.
In a mixing bowl, combine brown sugar, vinegar, soy sauce, Worcestershire sauce, mushroom soup, half of the onion soup mix, and broth. Mix well and pour over the roast.
Put the carrots and potatoes in a resealable plastic bag. Sprinkle the remaining half of the dry onion soup mix over the vegetables and shake the bag until they are coated. Add the vegetables to the roast in the crock pot half way through the cooking time.
Place lid on the slow cooker and cook on high heat for 5-6 hours, or on low heat for 10-12 hours.