Cut the meat into 1-inch cubes, discarding any large pieces of fat. Fat marbled within the meat is fine.
Season the beef with black pepper, garlic salt, and celery salt, tossing to coat evenly. Sprinkle flour over the meat and toss again to ensure even coverage.
Heat 3 tablespoons of olive oil in a large skillet over medium-high heat. Add the beef in batches, ensuring not to overcrowd the pan. Sear each side for about 45 seconds until browned. Add more oil as needed between batches. Transfer the seared beef to the slow cooker.
Lower the heat to medium and melt 1 tablespoon of butter in the same skillet. Add the diced onions and sauté for 5 minutes. Stir in the minced garlic and cook for an additional minute. Deglaze the pan with a splash of wine, using a silicone spatula to scrape up the browned bits, which will enhance the stew's flavor. Transfer the onion and garlic mixture to the slow cooker.
Add the remaining ingredients to the slow cooker, except for the peas, cornstarch mixture, and the 2 tablespoons of cold butter.
Cook on low for 7½ to 8 hours or on high for 3½ to 4 hours, until the vegetables are soft and the potatoes are fork-tender.
Stir in the peas during the last 15 minutes of cooking. Remove the bay leaves and the rosemary stem before serving.
If a thicker consistency is desired, mix ¼ cup of cold water with 3 tablespoons of cornstarch. Gradually add this mixture to the stew while stirring, allowing it to thicken. It will continue to thicken as it stands.
Once cooking is complete, turn off the heat and swirl in 2 tablespoons of cold butter to create a smooth, velvety texture. This technique, known as "Monter au Beurre," is a classic chef's method.
For an even richer color, you can add 3 drops of Gravy Master as an optional finishing touch.