Start by bringing a large pot of salted water to a rolling boil over high heat. Add pasta shells and cook for 7-9 minutes, just shy of al dente. Reserve 1 cup of cooking water before draining.

In a large skillet, melt the butter over medium heat, then pour in the evaporated milk and bring to a gentle simmer, stirring frequently.

Whisk in the pumpkin puree, mustard, and nutmeg into the simmering milk until well combined and creamy.

Gradually add the cheddar and Parmesan cheese into the sauce, stirring continuously until melted and creamy.

Fold the drained pasta shells into the cheese sauce and add reserved cooking water as necessary for the desired consistency.

Season with salt and pepper, serve immediately and garnish with chives if desired.