Mashed Potatoes with Pumpkin and Greens

Name: Mashed Potatoes with Pumpkin and Greens
Ingredients: 2 pounds Yukon Gold potatoes, peeled and cut into 2-inch pieces (5 cups); 1 head garlic, halved lengthwise; Coarse salt and freshly ground pepper; 3 pounds pumpkin or orange-fleshed squash, such as calabaza, peeled and cut into 2-inch pieces (8 cups); ½ stick unsalted butter; ¼ cup extra-virgin olive oil; 1 medium yellow onion, coarsely chopped (2 cups); 2 bunches rainbow Swiss chard (1 ½ pounds total), leaves and stems separated, leaves cut into 2-inch pieces; ½ cup whole milk; Pinch of freshly grated nutmeg