Put the butter and oil in a large pot over medium-high heat.
When the butter melts, add the leeks and cook, stirring occasionally, until soft and translucent, 5 to 7 minutes.
Add the nutmeg and cinnamon along with some salt and pepper and stir for 30 seconds or so, then add the apples and squash and stir to coat them in the oil and spices.
Add the stock, turn the heat to high, cover, and bring to a boil.
Reduce the heat to medium-low and simmer, adding more stock if necessary to keep the squash and apples covered with liquid.
When the squash and apples are soft enough to be pierced easily with a fork, turn off the heat, add the yogurt, and purée with an immersion blender.
Taste, add more salt if necessary, and serve hot, at room temperature, or cold.
