Cut beef into 1-inch cubes, discarding any large fat pieces. Season with pepper, garlic salt and celery salt, then toss with flour to coat.
Heat olive oil in a skillet over medium-high heat. Working in batches, sear beef cubes for 45 seconds per side until browned. Transfer to slow cooker.
Reduce heat to medium and melt 1 tablespoon butter. Cook onions for 5 minutes, then add garlic for 1 minute until fragrant. Deglaze pan with a splash of wine, scraping up any browned bits. Transfer to slow cooker.
Add remaining ingredients except peas, cornstarch slurry, and 2 tablespoons butter to slow cooker. Cook on low for 7 1/2-8 hours or high for 3 1/2-4 hours.
In the last 15 minutes, add peas. Remove bay leaves and rosemary stem.
For a thicker gravy, stir together the cold water and cornstarch, then slowly whisk into the stew.
Turn off heat and swirl in remaining 2 tablespoons cold butter until melted. Add browning sauce if desired.
Serve stew warm over mashed potatoes, egg noodles or with crusty bread.