Peel and finely dice the onion and garlic. Drain the tomatoes, collecting 1 tablespoon of oil. Bring 3-4 l salt water (1 teaspoon salt per litre) to the boil. Heat the oil and fry the onion, garlic and tomatoes. Add frozen spinach, 1⁄4 l water and stock. Bring to the boil and simmer covered for about 8 minutes.
Add the noodles to the boiling salted water and cook according to the instructions on the packet. Chop walnuts coarsely, grate cheese finely. Season sauce with salt and pepper. Drain noodles and mix with the sauce. Sprinkle with nuts and parmesan.