Clean and slice the mushrooms. Divide the thyme into small twigs and fry in oil with the mushrooms in a coated pan. Melt the fat, sauté the flour in it and deglaze with milk and vegetable stock.
Simmer the sauce over a mild heat for 10 minutes. After 5 minutes add the peas. Cook the pasta in salted water until al dente according to the instructions on the packet. Cut ham into small cubes. Rinse lemon hot and peel thinly.
Cut the skin into thin strips, add the ham and mushrooms to the sauce and bring to the boil. Remove the sauce from the stove, stir in yoghurt and season with salt, pepper and lemon juice. Drain the pasta and serve with the sauce.
Garnish as desired with lemon slices and thyme.