Preheat oven to 375°. Toss potatoes with 2 tbsp. oil and 1/2 tsp. each salt and pepper. Arrange in a single layer on a baking pan and bake until golden brown, 45 minutes.
Bring a medium saucepan of water to a boil and add remaining salt. Add green beans and cook until tender, about 5 minutes. Drain and rinse with cold water.
In a large bowl, whisk together vinegar, mustard, and remaining oil. Add potatoes and beans and toss to coat. Stir in tomatoes and olives; top with feta and basil.
Note: Nutritional analysis is per serving.