
Cut meat into 1-inch cubes, discard any large pieces of fat.
Sprinkle beef with black pepper, garlic salt, and celery salt. Toss to coat.
Sprinkle flour over the meat and toss again.
Heat olive oil in a large skillet over medium-high heat. Add the meat in batches, don’t overcrowd the pan. Brown on each side for 45 seconds. Add more oil if needed. Transfer to slow cooker.
Reduce heat to medium and melt 1 tablespoon of butter. Add onions and cook for 5 minutes. Add garlic and cook for 1 minute. Add a splash of wine and use a spatula to loosen brown bits in the skillet. Transfer the onions, garlic, and remaining wine to the slow cooker.
Add all remaining ingredients except for the peas, corn starch mixture, and 2 tablespoons of cold butter.
Cook on low for 450 minutes or high for 210 to 240 minutes, until the vegetables are softened and the potatoes are fork tender.
Add the peas during the last 15 minutes of cooking. Remove the bay leaves and rosemary stem.
Optional: To thicken, combine cold water with cornstarch. Slowly add it to the stew, stirring to incorporate.
Turn off the heat. Swirl in 2 tablespoons of cold butter for a smooth, velvety finish.