Put spaghetti in boiling salted water and cook for 10-12 minutes. Peel and finely chop the onion. Heat 1 tablespoon of fat in a pot and fry the onion cubes in it until transparent. Add crème fraîche and milk and let it boil down a little.
Grate cheese and add it while stirring, season with salt and pepper. Keep warm. Wash, clean and slice the tomatoes. Heat the remaining fat in a pan, fry the tomato slices briefly and season with salt and pepper.
Roast the sunflower seeds in a dry pan until golden brown. Rinse cress under running water. Drain spaghetti on a sieve, arrange on plates with tomatoes and sauce. Garnish with cress and sunflower seeds.