Begin by bringing a large pot of salted water to a rolling boil over high heat. Add the pasta shells and cook until just shy of al dente, about 1-2 minutes less than the package instructions. Drain the pasta, reserving 1 cup of the starchy cooking water for later, and set it aside.

In a large skillet, melt the butter over medium heat until it foams slightly without browning.

Pour in the evaporated milk, gently stirring as it comes to a simmer. Keep the heat moderate to avoid boiling.

Whisk in the pumpkin puree, yellow mustard, and nutmeg. Stir until well combined and creamy.

Gradually add the grated sharp cheddar cheese and grated parmesan cheese to the skillet, stirring until fully melted and smooth.

Add the drained pasta shells to the cheese sauce, gently folding until well coated. If the sauce is too thick, incorporate the reserved pasta cooking water until desired consistency.

Taste and add salt as needed. Serve immediately, garnished with finely chopped chives.