Sear beef (optional): In a large skillet, heat olive oil over medium-high heat. Pat beef cubes dry and season with salt and pepper. Sear in batches until browned on all sides (about 3–4 minutes per batch). This step adds depth but can be skipped for convenience.
Layer in slow cooker: Transfer seared (or raw) beef to a 6-quart slow cooker. Add diced potatoes, carrots, celery, onion, and garlic.
Add liquids and seasonings: Stir in beef broth, tomato paste, Worcestershire sauce, thyme, rosemary, and bay leaf. Mix well.
Cook: Cover and cook on low for 7–8 hours or on high for 4–5 hours. Beef should be fall-apart tender and potatoes soft.
Remove bay leaf: Discard the bay leaf.
Thicken (optional): If the soup is too thin, stir in cornstarch slurry and cook on high for 15–20 minutes until thickened. Alternatively, remove 1 cup of cooked potatoes, mash them, and stir back into the soup.
Add cream (optional): Stir in heavy cream, half-and-half, or sour cream for richness. Taste and adjust seasoning with salt and pepper.
Serve: Ladle into bowls and garnish with fresh parsley or chives if desired.