NOTE: MARINATE SALAD IN REFRIGERATOR FOR 30 MINUTES OR LONGER, THEN RE-SEASON WITH FINE SEA SALT AND RE-TOSS BEFORE SERVING.
PEEL and slice red onion into 1/4" rings; PLACE onion slices in a bowl of cold water.
REFRIGERATE bowl of water with onions until needed (the cold soak will diffuse sharpness and/or bitterness).
PREPARE DRESSING: Finely grate 1/4 teaspoon fresh lemon zest. Knead and cut lemons into halves to juice, yielding 1 1/2 tablespoons juice. Whisk together all ingredients in a small mixing bowl. Set aside at room temperature to allow flavors to infuse. FOR A TANGIER DRESSING: Add up to 2 1/2 - 3 tablespoons extra red wine vinegar, plus sugar to balance acidity. FOR A LIGHTER DRESSING: Replace some of the olive oil with kalamata olive brine before adding sea salt, then adjust salt according to taste.
PREP the remaining vegetables to specifications; DICE the feta cheese into 1-inch cubes.
REMOVE onions from water and blot dry.
COMBINE all ingredients except feta cheese in a large bowl.
POUR the dressing over vegetables and gently toss until evenly blended.
SPRINKLE top of salad with feta cheese (if desired you can gently fold feta with salad to moisten with dressing).