Meanwhile, make the sauce. In a large skillet, heat the olive oil over medium-high heat and add the onion. Sauté, stirring until the onions sizzle, then reduce heat to medium-low. Cook for at least 10 minutes. When the onions are soft and golden, add the garlic and sage, and stir for a few seconds, until fragrant. Stir in the pumpkin, vegetable stock, wine, salt and pepper and whisk to mix. Turn the heat to medium-high and bring to a boil, then reduce to medium and simmer vigorously, stirring frequently, about 5 minutes or until thickened. While the mixture is still hot, whisk in the yogurt and Parmesan cheese, then remove from heat.