Trim stew meat into 1-inch cubes, removing any large fat pieces but keeping marbled fat intact.
Season the beef evenly with black pepper, garlic salt, and celery salt; then dust it with flour and toss well.
Heat 3 tablespoons olive oil in a skillet over medium-high heat, and sear the beef in batches for about 45 seconds on each side; add more oil if necessary. Transfer browned beef to the slow cooker.
Lower the skillet to medium heat and melt 1 tablespoon of butter; cook diced onions for 5 minutes until softened.
Add minced garlic and sauté for 1 minute, then pour in a splash of wine to deglaze the pan, scraping the flavorful bits off the bottom.
Transfer the onion, garlic mixture and remaining wine into the slow cooker.
Add beef broth, beef bouillon cubes, Worcestershire sauce, tomato paste, carrots, potatoes, bay leaves, and rosemary to the slow cooker, excluding peas, cornstarch mixture, and cold butter.
Cook on low for 7½ to 8 hours or on high for 3½ to 4 hours until vegetables are tender and potatoes can be easily pierced with a fork.
Stir in frozen peas during the last 15 minutes of cooking, then remove the bay leaves and rosemary stem.
If desired, blend cold water with cornstarch and gradually stir into the stew to thicken, allowing it to set.
Turn off heat and gently mix in 2 tablespoons of cold butter to create a smooth, velvety finish.
Optionally, add 2-3 drops of Gravy Master for a deeper, richer color.