Toss the roast chunks in the flour, salt and pepper until coated. Heat half the oil in a sauté pan and cook the beef in batches until browned, making sure not to overcrowd.
Remove the meat and place in the slow cooker. Heat the remaining oil and add the tomato paste and garlic and cook for 1 minute, then deglaze the pan with the beer.
Add the carrots, celery, leeks, potatoes, beef bouillon, beef stock and Worcestershire sauce to the slow cooker. Stir well.
Pour the beer and tomato paste into the slow cooker. Add the thyme sprigs and cover tightly with the lid.
Turn on high for 4 hours or low for 6 to 8 hours. When the meat is pull apart tender, the stew is ready. Serve with a sprinkle of parsley, if desired.
Notes
Storage:
Store in an airtight container in the refrigerator for up to 5 days. May be frozen for up to 3 months.