Prepare vinaigrette just before use. Through the feed opening of a running blender
, add 1 at a time: champagne
vinegar, lemon juice
, chopped mint
, salt and pepper. Leaving the blender running, add the oil in a slow stream. Adjust seasoning, to taste, with salt and pepper.
Toss together arugula and onion
. Fold in watermelon
. Gently stir in enough of the vinaigrette to coat. Top with feta cheese
. Serve with any extra dressing
on the side.