Cut beef into 1-inch cubes and remove excess fat.
Season with black pepper, garlic salt, and celery salt.
Toss beef with flour until evenly coated.
Heat olive oil in a skillet over medium-high heat.
Brown the beef in batches, about 45 seconds per side. Transfer to slow cooker.
In the same pan, melt 1 tablespoon butter and cook onions for 5 minutes.
Add garlic and cook for 1 minute.
Pour in grape juice or broth and scrape up browned bits.
Transfer mixture to the slow cooker.
Add remaining ingredients except peas, cornstarch mixture, and remaining butter.
Cook on low for 7½–8 hours or high for 3½–4 hours.
Add peas during the last 15 minutes.
Optional: Stir in cornstarch mixture to thicken.
Turn off heat and swirl in remaining butter for a smooth finish.
This Slow Cooker Beef Stew is best served hot with crusty bread or on its own as a complete meal.