Fry the onion, mince and herbs for 5 minutes to brown. Add the tomato puree, chopped tomatoes and stock. Bring to the boil, cover and simmer for 10 minutes.
Add the kale and cook for a further 5 minutes. Season.
Meanwhile, cook the spaghetti in boiling water for 10 minutes until tender, drain and serve with the Bolognese sauce.