First, cook the pasta according to the package directions. Since it will keep cooking in the oven later, you'll want it slightly firm—so knock about 2 minutes off the suggested cook time.
Preheat your oven to 180° C and get ready to make that cozy cheese sauce!
Making the Cheese Sauce:
Cut the cheese into rough chunks and place in the mixing bowl. Grate for 8 sec / speed 10. Transfer to a bowl and quickly rinse the mixing bowl.
Add the butter to the mixing bowl and melt for 2 min / 60° C / speed 2.
Add the flour to create your roux and cook for 3 min / 100° C / speed 1.
Set the timer to 6 min / 90° C / speed 3 and slowly pour the milk and heavy cream through the opening in the lid. Pop the lid attachment back on.
When the time is up, add the grated cheese, mustard, nutmeg, salt, and pepper, then stir for 2 min / 100° C / speed 1 until velvety smooth.
Assembly & Baking:
Pour the cheesy sauce all over the macaroni and toss until every noodle is nicely coated.
Transfer the mixture into a baking dish and smooth the top into an even layer.
Melt the butter in the microwave or in your rinsed Thermomix® (2 min / 60° C / speed 2) and toss together with the panko breadcrumbs.
Scatter the buttery crumb mixture evenly over the pasta and bake on the upper-middle rack for 25-30 minutes until golden and bubbly.