For the pickled onions: Bring the vinegar to a boil in a small saucepan. Add the sugar and then pour over the red onions. Let cool, then refrigerate overnight. Strain the next day.
For the salad: Toss the greens, tomatoes, feta, green onions and pickled onions in a bowl.
For the dressing: Whisk together the vinegar, mustard, oil and some salt and pepper.
Add the dressing to the salad, toss and plate.