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.
Taking the recipe at face-value, I found the results fairly bland and unsatisfying. I made everything exactly as stated, but would double the vinaigrette (or halve the spinach) in the future - the spinach soaked up and overpowered the other flavors. Feta is also too dry of a cheese for this recipe; a melty mozzarella or gouda would have been much more gratifying. I don't see myself repeating this recipe.