Trim fat and cut the beef into 1-inch cubes, keeping any marbled fat within the meat.
Season the beef pieces evenly with black pepper, garlic salt, and celery salt, then dust with flour and toss well.
Warm 3 tablespoons of olive oil in a skillet over medium-high heat; cook the beef in small batches to brown each side for about 45 seconds without overcrowding. Add oil as necessary and move browned meat to the slow cooker.
Lower the skillet heat to medium and melt 1 tablespoon of butter; sauté diced onions for 5 minutes, add the minced garlic and cook another minute.
Pour in a splash of wine and scrape up browned bits from the pan with a spatula to release flavor, then transfer the onion, garlic, and wine to the slow cooker.
Add the rest of the ingredients except the peas, cornstarch mixture, and 2 tablespoons cold butter into the slow cooker.
Set the cooker to low and let it cook for 7 ½ to 8 hours or choose high and cook for 3½ to 4 hours until veggies are tender and potatoes are easily pierced.
Stir in the frozen peas during the last 15 minutes of cooking, then remove the bay leaves and rosemary.
If thicker stew is desired, mix cold water with cornstarch and gradually blend into the stew, stirring well. It will thicken as it sits.
Turn off heat and swirl in the remaining 2 tablespoons cold butter to give the stew a smooth, velvety texture. Optionally, add a few drops of Gravy Master for deeper color.