Preheat the oven to 375°F.

In a large saucepan, bring 2-3 quarts of water to a rolling boil and add a pinch of salt. Add 1½ cups of short tubular pasta and cook for about 12-15 minutes or until just past al dente. Drain pasta, reserving at least 1 cup of the pasta water.

Melt 2 tablespoons of butter in a large sauté pan with high sides.

Once the butter has melted, whisk in the all-purpose flour, salt, and mustard powder, keeping everything moving until a roux forms and begins to darken to a light amber.

To the roux, add 2-3 tablespoons of pasta water, one tablespoon at a time, until it is smooth, velvety, and slightly thin.

Add ½ cup of the combined white cheddar and Monterey Jack cheeses into the sauté pan with the roux, stirring to combine. Once fully incorporated, stir in the remaining ¾ cup of cheese.

Once all the cheese is melted, add the whipping cream and additional pasta water, if necessary, to get the sauce to a smooth but slightly runny consistency.

Add the cooked and drained pasta to the cheese sauce and stir to get it evenly coated.

In a medium bowl, mix the pulled pork with the BBQ sauce until well combined.

In a lightly-greased 8x8 casserole dish, add the macaroni and cheese in an even layer. Sprinkle with additional shredded cheese if desired. Top evenly with the pulled pork mixture.

Bake uncovered for 15-18 minutes, or until the dish is heated through and the sauce is bubbling around the edges.

Allow the mac and cheese to sit for at least 5-7 minutes before serving. Drizzle additional BBQ sauce on top if desired, and garnish with sliced green onions if using.