Steam the asparagus spears in a basket-style steamer over boiling water until tender, 5 to 10 minutes. Drain and remove to a serving dish; keep warm.
Meanwhile, melt half of the butter in a large skillet over medium-high heat. Once melted (it's ok if it begins to brown), stir in the sliced mushrooms, and cook until they brown, soften, and begin to release their liquid; pour into a serving dish, and set aside. Melt the remaining butter in the skillet, and stir in the onions. Cook until the onions soften and turn translucent, about 3 minutes. Season with garlic powder, basil, salt and pepper. Stir in the chopped pecans, and cook for a minute more.
Sprinkle the onion mixture with half of the Parmesan cheese, and stir in the reserved mushrooms. Pour over the asparagus in the serving dish and sprinkle with remaining cheese.