Use a mandoline or vegetable peeler to slice the squashes and carrots into super-thin slices. This gives them a lovely appearance and lots of surface area to absorb the piquant herb vinaigrette.
For the Vinaigrette, mix all ingredients in small bowl with wire whisk until well blended. Set aside.
Trim squash ends. Slice yellow squash lengthwise into ribbons with a vegetable peeler or mandoline, discarding outside ribbons and core. Slice zucchini crosswise into thin round slices with a knife, vegetable peeler or mandoline. Slice carrots lengthwise into ribbons with a vegetable peeler or mandoline.
To serve, divide squash and carrots among each salad plate. Top with radishes and onion. Serve with Vinaigrette on the side.