Cut the stew meat into 1-inch cubes, discarding any large pieces of fat. Sprinkle with black pepper, garlic salt, and celery salt. Toss to coat, then sprinkle flour over the meat and toss again.
Heat olive oil in a large skillet over medium-high heat. Add the meat in batches, browning on each side for 45 seconds. Transfer the browned meat to the slow cooker.
Reduce the heat to medium and melt 1 tablespoon of butter in the skillet. Add diced onions and cook for 5 minutes, then add minced garlic and cook for 1 minute more. Use a silicone spatula to scrape the bottom of the pan to incorporate any browned bits. Transfer the onion-garlic mixture to the slow cooker.
Add all remaining ingredients (except peas, cornstarch mixture, and remaining 2 tablespoons of cold butter) to the slow cooker.
Cook on low for 7 ½ to 8 hours or on high for 3 ½ to 4 hours, until the vegetables are softened and the potatoes are fork-tender.
Add the frozen peas during the last 15 minutes of cooking (optional). Remove the bay leaves and rosemary stem.
To thicken the stew, combine cold water and cornstarch, then slowly add it to the stew, stirring to incorporate. It will continue to thicken upon standing.(Optional:)
Turn off the heat and swirl in the remaining 2 tablespoons of cold butter for a smooth, velvety finish.
Serve the slow cooker beef stew hot with warm biscuits, cornbread, or over mashed potatoes.