In a large (12-inch) pot or Dutch oven, such as Le Creuset, heat the remaining 2 tablespoons of butter and 1 tablespoon olive oil over medium heat. Add the onions and garlic and cook, stirring occasionally, for 6 to 8 minutes, until tender. Stir in the squash, nutmeg, 2 teaspoons salt, and ¾ teaspoon pepper, cover, and cook for 10 minutes, stirring occasionally.