Toss the beef pieces with the flour, salt, and pepper in a bowl until evenly coated.
Heat the olive oil in a large skillet over medium-high heat. Add the beef in batches and brown on all sides, then transfer it to the slow cooker.
Add the celery root, carrots, onion, and garlic to the slow cooker.
In a small bowl, whisk together the ale, beef broth, tomato paste, and thyme. Pour the mixture over the meat and vegetables.
Cover and cook on low for 7 to 8 hours, until the beef is fork-tender.
Stir in the frozen peas during the last 20 minutes of cooking so they heat through but keep their color and shape.