Heat 1 tbs oil in a large frying pan over high heat. Add one-third of the beef and cook, turning, for 3 minutes or until browned. Transfer to a 5.5l slow cooker. Repeat with remaining oil and beef.
Add onion to same pan. Reduce heat to low and cook for 8 minutes or until softened. Add garlic, carrot and celery, then cook for a further 8 minutes or until tender. Add tomato paste and cook, stirring, for 2 minutes. Add wine and simmer for 3 minutes or until reduced by half. Transfer mixture to slow cooker. Add tomatoes, stock, thyme, bay leaf and sugar to slow cooker, then stir to combine. Cover with lid, then cook on low for 8 hours or until beef is very tender.
Add pasta to slow cooker, gently stirring into ragu to combine. Cook on low for 3 minutes or until pasta is tender. Divide ragu mixture among bowls. Serve topped with parmesan and parsley.