Cut the meat into 1-inch cubes and trim away any large pieces of fat, leaving the marbled bits in place. The smaller, even pieces cook more evenly and the marbling helps the beef stay tender.
Season the beef with black pepper, garlic salt, and celery salt, then toss it with the flour until coated. A light flour coating helps the meat brown and gives the stew a better base for thickening later.
Heat 3 tablespoons olive oil in a large skillet over medium-high heat and brown the beef in batches for 45 seconds per side, adding more oil if needed, then transfer it to the slow cooker. Look for browned edges instead of gray meat, because that color means real flavor has developed.
Lower the heat to medium, melt 1 tablespoon of butter, and cook the onions for 5 minutes before adding the garlic for 1 minute. The onions should soften and smell sweet, and the garlic should turn fragrant without browning hard.
Pour a splash of wine into the skillet and scrape up the browned bits with a silicone spatula, then transfer the onions, garlic, and remaining wine to the slow cooker. The pan should loosen easily and the liquid will turn darker as it picks up all that built-up flavor.
Add the beef broth, bouillon cubes, Worcestershire Sauce, tomato paste, carrots, potatoes, bay leaves, and rosemary to the slow cooker. Once stirred together, the stew should look brothy and well seasoned, with the vegetables tucked into the liquid.
Cover and cook on low for 7 ½ to 8 hours or on high for 3½ to 4 hours. The stew is ready for the final steps when the carrots are softened and the potatoes are fork tender.
Stir in the peas during the last 15 minutes of cooking, then remove the bay leaves and rosemary stem. The peas stay bright and sweet this way instead of fading into the background.
If you want a thicker gravy, stir together 1/4 cup Cold Water + 3 tablespoons Corn Starch (Optional), then slowly mix it into the stew. The broth will go from loose to glossy and will keep thickening a bit as it stands.
Turn off the heat and swirl in the remaining 2 tablespoons of cold butter, then add 2-3 drops Gravy Master if desired. The gravy should look smooth and shiny, clinging gently to the beef and vegetables instead of running thin.