Butternut Squash Soup

Name: Butternut Squash Soup
Ingredients: 3 pounds butternut squash (about 1 large or 2 small), peeled; 1 pound yellow onions (about 2 medium), peeled and quartered; 2 tablespoons extra-virgin olive oil; Kosher salt and freshly ground black pepper; 3 garlic cloves, smashed and peeled; 2/3 cup walnut pieces (or chopped walnuts); 3/4 ounce Parmesan, finely grated (about 6 tablespoons); 1 teaspoon apple cider vinegar; Whole-milk yogurt, for swirling on top (optional)