For the dressing, whisk together vinegar, lemon juice, both types of mustard, salt and pepper from the mill and then whisk in the oil and cream so that it becomes slightly creamy. If the dressing becomes too creamy, you can add some more broth.
Cut the chives into fine rolls. Wash the cucumber and cut it unpeeled into wafer-thin slices. Clean the mushrooms (do not peel, otherwise they will lose a lot of aroma) and cut into thin slices.
Lay out alternating slices of mushrooms and cucumber on 4 plates and drizzle with the dressing. Scatter the chives rolls, green and pink pepper over the top.