Heat the oil in a large skillet over medium-high heat. Add the beef in a single layer and brown on all sides without crowding the pan. Work in batches if needed. Transfer the browned meat to a 4–6 quart slow cooker.

In the same hot skillet, add the onions and cook over medium heat until softened, about 4–5 minutes. Add the crushed garlic and cook 30 seconds until fragrant.

Pour about 1/2 cup of the beef broth into the skillet and scrape the browned bits (fond) off the bottom with a wooden spoon. Take the skillet off the heat and carefully pour the deglazed liquid and onions into the slow cooker over the meat.

Add the potatoes, carrots and celery into the slow cooker on top of the beef.

In a measuring jug, whisk together the remaining beef broth, Worcestershire sauce, and flour until smooth. Pour this mixture over the meat and vegetables and stir gently to combine. Tuck in the thyme, rosemary, and optional bay leaf. Season with 1/2 teaspoon salt and fresh ground pepper to taste.

Cover and cook on LOW for 7–8 hours or on HIGH for 4 hours, until the beef is fork-tender and the vegetables are cooked through.

When ready to thicken, remove the lid and stir the stew. Mix 2 tablespoons cornstarch with 3–4 tablespoons cold water to make a smooth slurry. Stir the slurry into the hot stew.

Simmer on HIGH without the lid for 15–30 minutes, stirring occasionally, until the gravy has thickened to your liking. Taste and adjust seasoning before serving.