In a small bowl, combine the lemon zest with the lemon juice and orange zest. Squeeze the juice from the zested orange into the bowl. Whisk in the mustard and olive oil and season the vinaigrette with salt and pepper.
Using a sharp knife, remove the peel and bitter white pith from the remaining 2 oranges and the grapefruits. Working over a plate, cut between the membranes to release the segments.
In a large bowl, toss the mesclun with the chopped parsley and the citrus vinaigrette; season the salad with salt and pepper. Add the citrus segments to the greens and serve right away.