Pierce each potato several times with a fork. Microwave at HIGH 13 minutes or until tender, turning after 7 minutes.
Combine oil and next 4 ingredients (through salt) in a bowl, stirring well with a whisk. Heat vinaigrette in a large skillet over medium-high heat. Add onion and garlic; sauté 2 minutes. Gradually add spinach; toss until wilted. Divide spinach among potatoes. Top each with 2 tablespoons crumbled feta cheese.