1) Preheat your oven to 400 degrees and prep all your vegetables: chop onion, garlic, and thaw and roughly chop the spinach. When you thaw your spinach, it’s always a good idea to press out some of the moisture with a few paper towels.
2) Cook the pasta according to the package.
3) For the sauce, melt your butter over medium high heat. Once it’s melted add the onions and garlic. Cook until they are soft, then add your flour flour to the mix. Whisk this all together to make a roux. Cook it for a few minutes until the flour takes on a light tan color.
4) Start adding the milk to the pan slowly, whisking constantly. Work slowly and whisk out any lumps that form. This works a lot easier if your milk is warm, but you can do it with cold milk also.
5) When all your liquid is incorporated, continue to whisk over low heat. Remember, there will be some lumps in this sauce because you have onions and garlic in there.
6) Once your sauce is the consistency of a light gravy, add in the cheeses and continue to whisk until they melt. Add any spices you are using at this point.
7) In a large bowl, combine the cheese sauce with the pasta and spinach. Season with salt and pepper.
8) Lightly butter a baking dish and pour in your cheese fusilli. Coat with bread crumbs and grated Parmesan.
9) Bake this whole thing at 400 degrees for 30-40 minutes until the edges are nice and crispy.
10) Let it cool for five minutes before serving.