Step 1: Add the beef and vegetables - Add the stew beef, diced onion, carrots, potatoes, and celery directly into the slow cooker. Spread everything out evenly so the vegetables cook at the same rate.
Step 2: Mix the gravy base - In a mixing bowl, whisk together the beef broth, tomato paste, Dr. Pepper, brown gravy mixes, and beefy onion soup mix. Take a minute to really whisk it well so there are no lumps.
Step 3: Combine everything - Pour the liquid mixture over the beef and vegetables in the slow cooker. Gently stir just until everything is coated. No need to overdo it.
Step 4: Cook low and slow - Cover the slow cooker and cook on low for 6 hours or high for 4 hours. The stew is ready when the beef is tender and the vegetables are soft.
Step 5: Serve and enjoy - Give the stew a gentle stir, ladle it into bowls, and serve hot. This is one of those meals that does not need much else to feel complete.