Clean the mushrooms and the leek, cut both into slices or rings (not too thin). Rinse the chives under running water and cut into fine rolls.
Preheat the oven. Heat the oil in a pan and fry the mushrooms and leeks for 3-4 minutes, then season with salt and pepper and drain on paper towels. Heat the butter in a saucepan, stir in the flour and let it sweat for about 1 minute, pour in the milk and stock and bring to the boil while stirring. Cut the chest cheese into cubes and stir into the sauce until it has dissolved, season with salt and pepper. Finally add the chives, stir the vegetables into the sauce and place in a baking dish. Sprinkle the grated Gouda cheese on top and put in the oven at 200 ° C for 20 minutes.