03While onions cook, heat a sauce pot over medium heat. Add butter, melt and whisk in flour. Cook 1 minute then whisk in stock and milk and bring to a bubble. Thicken a few minutes then season with salt and pepper. Melt in cheese stirring it in a figure-8 motion with a wooden spoon. Add nutmeg and mustard and adjust salt and pepper.04Drain pasta, toss with onions and stir in cheese sauce to coat. Top with crispy bacon and serve.