In a medium pot, melt the butter over very low heat. Add the flour and cook for 2 minutes.

Whisking briskly, pour the milk into the butter and flour roux in a slow stream. Cook the white sauce for 25 to 35 minutes until it slightly thickens. Whisk occasionally and whisk more frequently as the white sauce starts to simmer.

Remove the white sauce from the heat and whisk in the Parmesan, salt, and pepper.

Allow the cheese sauce to cool slightly while cooking the pasta and pork. Then, whisk in the egg yolks shortly before assembling the casserole.

Boil the fusilli pasta in salted water, as per the instructions on the package. Don’t overcook. Drain and rinse the fusilli with cold water to stop it from cooking more.

Combine flour, salt, and pepper. Add the pork and toss until coated.

In a frying pan, heat your olive oil over medium-low heat. Add the pork and stir fry for 5 to 10 minutes, until lightly browned.

Add the onion and let to cook for 10 minutes until the onion is tender.

Toss the pasta and pork until well distributed.

Preheat the oven to 350 degrees Fahrenheit.

Spoon the mixture into a 9×13-inch casserole dish. Pour the cheese sauce evenly over the top and use a spatula or spoon to work it down into the pasta.

In a bowl, combine mozzarella and Parmesan cheese. Sprinkle evenly over the pasta.

Bake for 45 minutes.