Warm 1 Tbsp. vegetable oil in a large skillet over medium-high heat. Add onions and mushrooms and cook, stirring frequently, until vegetables have softened slightly and mushrooms have released their water, about 7 minutes. Scrape vegetables into slow cooker. Warm remaining 1 Tbsp. vegetable oil in same skillet. Add beef and cook, stirring, about 5 minutes, until meat is browned on all sides (if skillet is too small, cook beef in batches, adding more oil if needed). Scrape beef into slow cooker.
Pour 1 cup of broth into skillet and stir with a wooden spoon to scrape up any browned bits on bottom of pan. Pour broth from skillet, remaining broth, carrots, tomatoes, Worcestershire sauce, thyme, garlic powder, barley and 1 tsp. salt into slow cooker. Stir, cover and cook on low until meat is tender, 6 to 7 hours. Season with salt and pepper and serve hot.