Using a mandolin or a very sharp knife, slice zucchini into very thin rounds. Overlap zucchini disks in 1 layer on a plate; season with salt and pepper. Drizzle over the olive oil and lemon juice and scatter with leeks. Using a mandoline or a vegetable peeler, shave very thin slices of Parmesan and place them over leeks. Garnish with mint leaves. Serve immediately.