Beans and Potatoes with Mint Pesto

Name: Beans and Potatoes with Mint Pesto
Ingredients: 1 1/2 pounds fingerling or other small potatoes, scrubbed; Salt; 1 pound string beans, stem ends trimmed, cut into 2-inch lengths; 1/2 cup fresh basil leaves, packed; 1 cup fresh mint leaves, packed; 1/2 cup spinach leaves, ribs removed; 1 tablespoon pine nuts; 1 small clove garlic, peeled; 5 tablespoons extra-virgin olive oil; 3 tablespoons freshly grated Parmesan cheese; 1 tablespoon unsalted butter, room temperature; 6 scallions, white parts only, thinly sliced