Cut beef into 1½-inch cubes, trimming away large fatty pieces. Place in a large bowl and season generously with black pepper, garlic salt, and celery salt. Toss to coat evenly, then sprinkle flour over meat and toss again until each piece has a light dusting.

Heat 3 tablespoons olive oil in large skillet over medium-high heat until shimmering. Working in batches of 8-10 pieces, add beef cubes in single layer without crowding. Brown each side for about 45 seconds until golden-brown crust forms. Add more oil between batches if needed. Transfer seared meat to 6-quart slow cooker.

Reduce heat to medium and add 1 tablespoon butter to same skillet. Add chopped onions and cook 5 minutes, stirring occasionally, until soft and translucent. Stir in minced garlic and cook 1 minute until fragrant. Pour in about ¼ cup wine and use silicone spatula to scrape up flavorful brown bits from bottom of pan. Transfer mixture to slow cooker.

Add remaining wine, beef stock, tomato paste, Worcestershire sauce, bouillon cubes, carrots, potatoes, bay leaves, and rosemary to slow cooker. Stir to combine. Cover and cook on low for 7½ to 8 hours, or on high for 3½ to 4 hours, until vegetables are tender and potatoes break apart easily with fork.

During last 15 minutes of cooking, stir in frozen peas. Remove and discard bay leaves and rosemary stem. For thicker stew, whisk together cold water and cornstarch in small bowl to create smooth slurry. Slowly drizzle into stew while stirring gently, and let cook for remaining time to thicken.

Turn off heat once stew reaches desired consistency. Swirl in 2 tablespoons cold butter until melted completely. For richer color, add 2-3 drops Gravy Master if desired. Serve hot in deep bowls.