Coat the beef with flour and salt by tossing everything in a sealable bag.
In a large skillet over medium-high heat, heat the oil until shimmering. Sear the beef with Worcestershire sauce until evenly browned, then transfer to a slow cooker without cleaning the skillet. Add carrots, potatoes, parsley, and pepper to the slow cooker.
Mix the boiling water and soup mix in a small bowl, then pour into the slow cooker.
In a skillet over medium-high heat, melt the butter until sizzling. Saute the onion and garlic until tender. Transfer them to the slow cooker. In the same skillet, combine wine and mushrooms, cooking until mushrooms soak up the wine. Add this mixture to the slow cooker.
1. Cover the slow cooker and set it to High; cook for 1 hour.
2. Lower the heat to Low and continue cooking until the beef is fork-tender, 6 to 8 hours.
3. Combine warm water and cornstarch, then stir into the stew.
4. Simmer uncovered until the stew thickens, about 15 minutes.