Preheat oven to 350°. Melt butter in a large frying pan over medium-high heat. Add onions, garlic, and salt. Cook, stirring occasionally, until onions are soft, about 3 minutes. Stir in sage and cook until fragrant, about 1 minute. Stir in nuts. Set aside.
In a large bowl, combine yogurt, eggs, and 1/2 cup parmesan. Stir in nut mixture. Divide stuffing among squash halves, sprinkle with more parmesan, and bake until tender when pierced with a fork and tops are browning, about 45 minutes.
Note: Nutritional analysis is per squash half.