Heat a 12-inch skillet over medium-high heat and add 2 tablespoons olive oil. Season 3 pounds stew meat with 1 ½ teaspoons salt and ½ teaspoon ground black pepper before adding it to the skillet in batches. Cook the beef just long enough to brown the outside before adding it to a 4-quart slow cooker.
Add 3 cups beef broth, 1 (2-ounce) packet onion soup mix, 1 tablespoon Worcestershire sauce, 1 cup diced onion, 2 cups sliced mushrooms, and 3 teaspoons minced garlic. Cover and cook on LOW for 5-6 hours or HIGH for 3-4 hours.
30 minutes before the beef is done cooking, mix ¼ cup water and 3 tablespoons cornstarch in a small bowl to make a slurry. Stir the slurry into the slow cooker, cover, and let it cook and thicken for the final 30 minutes.