If using fresh green beans, cook in a covered saucepan in a small amount of boiling water for 20 to 25 minutes or until crisp-tender. Drain.
Meanwhile, in a large saucepan cook pasta according to package directions, adding the mushrooms, frozen green beans (if using), and green onion during the last 4 to 5 minutes of cooking. Drain pasta and vegetables in a colander. Set pasta mixture aside.
For sauce, in the same large saucepan stir together yogurt, flour, mustard, nutmeg, and pepper. Cook and stir until thickened and bubbly. Stir in pasta mixture, cooked fresh green beans (if using), and pancetta or ham. Heat through. To serve, sprinkle with Parmesan cheese.