Brush the mushrooms, cut off the stems, cut into cubes and fry in the oil in a large saucepan. Clean and wash the leek and cut into 1/4 rings, peel and dice the onions. Once the water that has leaked out of the mushrooms has evaporated, brown them a little and then add the leeks and onions. Simmer on high heat for a few minutes until everything has taken some color. Then deglaze with vegetable stock, dissolve the sediment and simmer for about 20 minutes over low heat with the lid closed.
Add the cheese and melt. Puree the soup and season with salt and pepper.