Toss the beef, flour, and salt in a sealable bag until the beef is coated.
Heat the oil in a large skillet over medium-high heat. Combine the beef and Worcestershire sauce in the skillet; cook until the beef is evenly browned on all sides; transfer to a slow cooker, but do not clean the skillet. Add the carrots, potatoes, parsley, and pepper to the slow cooker.
Combine the boiling water and soup mix in a small bowl; add to slow cooker.
Melt the butter in the skillet over medium-high heat. Cook the onion and garlic in the melted butter until soft; transfer the onion and garlic to the slow cooker and return the skillet to the heat. Combine the wine and mushrooms to the skillet; cook until the mushrooms begin to absorb the wine; pour the mixture into the slow cooker.
Place the cover on the slow cooker and set to High; cook for one hour. Reduce heat to Low and cook until the beef is fork-tender, 6 to 8 hours. Whisk together the warm water and cornstarch; stir into the stew; cook uncovered until stew thickens, about 15 minutes.