Bring a pot of salted water to a boil. Add the pasta and cook according to the package directions, but for 1 minute less. Save 1 cup of starchy pasta water, drain the pasta, and rinse with cool water. Set aside.

Warm the milk in a small saucepan, turn off the heat, cover with a lid, and set aside.

Melt the butter in a large pot over medium heat. Sprinkle the flour over the melted butter and whisk until thick and light brown, about 1-2 minutes.

While whisking, pour in a third of the warm milk and whisk until smooth. Repeat with the next third and then the last third. Cook until the sauce simmers and thickens, about 1 minute.

Season with ¼ teaspoon salt, ¼ teaspoon black pepper, nutmeg, and mustard powder (if using).

Turn the heat to low, stir in the cream cheese until melted, then add the shredded cheese gradually until it melts into the sauce. Adjust seasoning as needed.

Stir in the drained and rinsed pasta. Take the pot off the heat, cover, and let it sit for 5 minutes. Serve, adding reserved pasta water to thin it out if necessary.