Pat the beef dry with paper towels, then season with salt and pepper.
Place a large heavy-bottomed skillet or pot over medium-high heat. When hot, add 2 teaspoons of the oil and swirl to coat the bottom. Add half of the beef in a single layer and cook until well browned on all sides, about 7 minutes.
Transfer to a 6-quart slow cooker. Repeat with 2 more teaspoons of the oil in the skillet and the remaining beef.
Add the remaining 2 teaspoons oil to the skillet and decrease the heat to medium. Add the onion and 1/2 cup of the beef broth. Cook, scraping up any browned bits from the bottom of the skillet with a wooden spoon, until the onion starts to soften, 6 to 8 minutes.
Add the garlic and cook, stirring occasionally, for about 1 minute.
Transfer the onion and garlic to the slow cooker. Stir in the carrots, celery, beer, tomato paste, bay leaves, thyme, and remaining 1/2 cup broth. Cover and cook on low for 8 hours.
Remove the bay leaves. Taste and season with salt and pepper. Serve over garlic mashed potatoes, if desired, and garnish with parsley or thyme, if using.