Cut the beef into uniform 1-inch pieces, removing any large deposits of fat while leaving fine marbling intact.
Season the beef evenly with black pepper, garlic salt, and celery salt. Toss thoroughly so the seasoning reaches all sides of the meat.
Dust the seasoned beef with flour and turn the pieces until they are lightly and evenly coated.
Warm 3 tablespoons of olive oil in a large skillet set over medium-high heat.
Place the beef in the skillet in batches, keeping the pieces spaced apart to prevent crowding.
Sear the beef for about 45 seconds on each side, adding additional oil as needed. Once browned, transfer the meat to the crockpot.
Lower the skillet heat to medium and melt 1 tablespoon of the butter.
Add the chopped onions and cook for about 5 minutes, until softened and translucent.
Stir in the minced garlic and cook for 1 additional minute, just until aromatic.
Pour a small splash of the Cabernet Sauvignon into the skillet and scrape the browned bits from the bottom with a silicone spatula.
Transfer the onion, garlic, and pan juices into the crockpot, making sure all loosened browned bits are included.
Add the remaining ingredients to the crockpot except the peas, cornstarch, cold water, the remaining 2 tablespoons of butter, and the optional Gravy Master.
Stir the stew ingredients together until well combined.
Cover and cook on low for 7½ to 8 hours, or on high for 3½ to 4 hours, until the vegetables are tender and the potatoes can be pierced easily with a fork.
During the final 15 minutes of cooking, stir in the frozen peas.
Remove and discard the bay leaves and the rosemary stem.
For a thicker consistency, whisk the cold water with the cornstarch until smooth to form a slurry.
Gradually pour the slurry into the stew while stirring, then allow the stew to stand briefly so it can thicken.
Turn off the heat once the stew reaches the desired texture.
Stir in the remaining 2 tablespoons of cold butter to create a smooth, glossy finish. Add the optional Gravy Master for a richer color.
Serve the crockpot beef stew hot.