Pat the 3 pounds beef sirloin tip roast dry. Rub with 1 teaspoon salt, 1 teaspoon black pepper, and 3 cloves garlic.
Heat 2 tablespoons olive oil in a skillet over medium-high heat. Brown the roast on all sides.
Place 1 medium onion, 3 medium carrots, 3 medium potatoes, and 2 stalks celery in the bottom of the slow cooker. Sprinkle 1 tsp dried thyme and 1 tsp dried rosemary over the vegetables.
Pour 2 cups beef broth, 2 tbsp Worcestershire sauce, and 12 ounces mushroom gravy over the veggies.
Place the seared roast on top of vegetables. Cover and cook on low for 6–8 hours or high for 4–5 hours, until meat is tender.
Mix 2 teaspoon cornstarch or flour with a small amount of cooking liquid and stir into slow cooker until desired consistency is reached.
Slice roast against the grain and serve with vegetables and mushroom gravy.