Pour flour and beef into a large ziploc bag and toss until all chunks are coated with flour.
Heat oil in a large skillet over medium-high heat. Add beef and cook, turning occasionally, until all sides are browned.
Place beef, potatoes, carrots, celery, onion, water, soup, Worcestershire sauce, A-1 sauce, browning sauce, bouillon granules, garlic, salt, and pepper in a 5- or 6-qt. slow cooker. Stir well to combine.
Cover and cook on low for 6-8 hours or until meat is tender.
Combine cornstarch and cold water in a small bowl until smooth; gradually stir into stew. Stir in peas and mushrooms (if desired). Cover and cook on high for 30 minutes or until thickened.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/slow-cooker-beef-stew/