Clean the mushrooms and cut into slices. Dice the onion. Cut the cucumber into very small cubes.
Heat some oil in a pan and fry the mushrooms. When they have turned a little brown, take them out again.
Heat some oil again in the same pan. Fry the onion, bell pepper and cucumber for about 5 minutes. Add red wine, vegetable stock and cream. Add the mushrooms again and simmer for 10 minutes. Mix the starch with 2 tablespoons of cold water. Add to the sauce and let it thicken. Season to taste with mustard and spices.