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.
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 it in the refrigerator until ready to serve. In a small bowl, combine all the ingredients for the Lemon Vinaigrette and whisk all together. Pour the Lemon Vinaigrette right before serving.
To Store
You can keep the leftovers in an airtight container and store in the refrigerator for up to 2 days. Keep the dressing in a separate container.