Season the beef. Pat the beef dry.
Sprinkle with salt and pepper. This helps the sear and boosts flavor.
Quick sear for flavor. Heat olive oil in a large skillet over medium-high. Sear the beef in batches for 1–2 minutes per side until browned.
Don’t overcrowd. Transfer to the slow cooker.
Sauté aromatics. In the same pan, melt butter. Add onion and mushrooms; cook 4–5 minutes until softened.
Stir in garlic for 30 seconds. Scrape in any browned bits with a splash of wine or broth.
Build the sauce. Whisk together beef broth, Worcestershire, tomato paste, soy sauce, thyme, and wine (if using). Pour over the beef and vegetables in the slow cooker.
Add the bay leaf.
Slow cook. Cover and cook on Low for 6–8 hours or High for 3–4 hours, until the beef is very tender.
Thicken the gravy. Whisk flour with 1/3 cup cold water (or cornstarch with 2 tablespoons cold water) to make a slurry. Stir into the slow cooker. Cook 15–20 minutes more on High until the gravy is glossy and slightly thick.
Adjust salt and pepper.
Finish and serve. Remove bay leaf. Sprinkle with chopped parsley. Serve over creamy mashed potatoes, buttered egg noodles, or fluffy rice.