Pierce potato a few times with a fork. Place on a microwave safe plate, and cook 10 minutes in the microwave on high, until tender but not mushy. Transfer potato to a baking dish, and bake 15 minutes in the preheated oven.
Melt butter in a saucepan over medium heat. Mix in onion. Cook and stir until tender. Mix in mushrooms. Season with salt. Reduce heat to low, cover, and let sit 5 minutes, or until mushrooms are tender. Serve potato topped with the mushrooms and yogurt.