1. Place potatoes in lightly salted water to cover in a large saucepan. Bring to a boil, reduce heat, and simmer 20 to 25 minutes or until tender when pierced with a fork. Drain, cool to room temperature, and quarter. Set aside.
2. Toss spinach with 1/2 cup Greek Vinaigrette in a large bowl. Transfer to serving platter.
3. Combine potato, onion, and next 4 ingredients in a large bowl. Toss gently with remaining 1/2 cup of Greek Vinaigrette. Arrange vegetables over spinach, and top with feta. Garnish, if desired.