Bring a pot of water to a boil and cook the spaghetti with a little salt in it according to package instructions until al dente. Drain through a sieve and leave to drain. For the sauce, peel and chop a small onion and sauté. Deglaze with the vegetable stock and add the milk. Add the tomato paste and stir with a whisk. Bring to a boil and thicken with sauce thickener as needed. Do not let the sauce simmer any longer. Then cut the fish fillet into pieces and let it simmer.
Season to taste with salt and a little sugar, refine with herbs if necessary. Arrange on plates with the spaghetti and serve.
