Season beef with salt and pepper. Heat 1 tablespoon oil in a large skillet on medium high heat. Add the beef to the pan, one half at a time so as to not crowd the pan. Sear the beef until browned on all sides. Transfer to a plate. Repeat with the remaining beef.
Add the onion to the skillet and cook for 5 minutes, stirring occasionally. Add garlic and cook for 1 minute. Stir in the flour until the onions are coated and cook for 1 minute more.
Transfer everything in the skillet to your slow cooker. Gradually pour in the bottle of Guinness. Stir in the beef stock, carrots, celery, tomato paste, bay leaf, thyme and beef.
Cook on low for 7 - 8 hours or high for 3 - 4 hours, until the celery and carrots are tender. Optionally: serve over mashed potatoes and garnish with parsley.