Bring 2-3 l salted water to the boil. Preheat oven (electric cooker: 225°C/circulating air: 200°C/gas: see manufacturer). Peel and chop the onion and fry in hot butter. Dust with flour and sweat lightly. Stir in 1/4 l water, stock and cheese, bring to the boil.
Simmer for about 5 minutes. Season with salt and pepper.
Wash the spinach. Cook the tortelloni according to the instructions on the packet. Collapse the spinach in pasta water, drain. Wash the tomato, cut into slices. Place in a casserole dish with tortelloni mix and sauce.
Bake in the oven for 10-15 minutes.