1. Preheat oven to 375°.
2. In a large skillet over medium meat, melt butter. Add onion and garlic and sauté for 2 mins.
3. Add flour and cook for 1-2 minutes. Slowly whisk in milk; cooking approximately 5-7 minutes or until thickened.
4. Whisk in salt, black pepper, nutmeg, pesto, and ½ of the Parmigiano. Add diced chicken.
5. Cook pasta according to the package directions, adding asparagus for the last minute of the cooking time. Drain and toss with oil.
6. Cover with foil and bake for 10 minutes.
7. Remove foil and bake an additional 5-7 minutes or until lightly browned.