Prepare the Beef: Cut your chuck roast into 1½-inch cubes, ensuring excess fat is removed. Season with black pepper, garlic salt, and celery salt, then coat with flour.

Brown the Beef: Heat olive oil over medium-high heat. Brown beef in batches for 45 seconds on each side, then transfer to the slow cooker.

Sauté the Onions: Melt butter, add onions, and sauté until soft. Stir in minced garlic and cook for an additional minute.

Deglaze the Skillet: Pour red wine into the skillet, scrape up browned bits, and allow to simmer for 2-3 minutes before transferring to the slow cooker.

Combine Ingredients: Add browned beef, mixed vegetables, beef stock, bay leaves, and rosemary to the slow cooker.

Slow Cook the Stew: Cook on low for 7½ to 8 hours or high for 3½ to 4 hours.

Add Peas: Stir in frozen peas during the last 15 minutes of cooking.

Thicken the Stew (if desired): Mix cornstarch with cold water and stir into the stew during the last 10 minutes.

Finish with Butter: Just before serving, swirl in a cold tablespoon of butter.