Place the mushrooms, onion, and garlic in a slow cooker. Pat the beef dry and season with salt and pepper. In a large skillet or Dutch oven, heat 1 tablespoon oil over medium-high heat. When the oil shimmers, add about half of the meat and cook until browned on all sides, about 10 minutes total. Transfer browned beef to the slow cooker. Repeat with remaining meat, adding additional oil as needed. Transfer the remaining browned meat to the slow cooker.
Add the gravy mix, beef broth, red wine, and Worcestershire sauce to the slow cooker. Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, or until the meat is tender. If the gravy is thinner than you would like at the end, just whisk in a cornstarch slurry to thicken. To do so, stir together 1 tablespoon of cornstarch with 2 tablespoons of cold water. Add this slurry to the slow cooker and cook on HIGH heat until the gravy thickens. Gradually add more cornstarch slurry, as needed, until the gravy reaches your desired consistency.
Serve with noodles, rice, or mashed potatoes. Garnish with chopped fresh parsley, if desired.