Slice the onion with a slicer.Soak the sliced onion in cold water for 15 minutes. Drain and squeeze water out. Pat dry with paper towels and set aside.Zest the lemon and keep this lemon for juice.Slice the other lemon with a slicer. If you press the lemon against the slicer, you can get nice thin slices.In a small bowl, combine all the ingredients for Lemon Vinaigrette and whisk all together.Spread mixed green salad on a plate and put sliced onion on top.Put smoked salmon and sprinkle additional pepper.Place some sliced lemons and keep in the fridge until ready to serve. Pour the Lemon Vinaigrette right before serving.