Bring a large pot of salted water to a boil. Add 1 pound of pasta shells and cook until just shy of al dente, about 1-2 minutes less than package instructions. Reserve 1 cup of cooking water and drain the pasta.

In a large skillet, melt 2 tablespoons of butter over medium heat. Once melted, pour in 12 ounces of evaporated milk and heat until it starts to simmer, stirring constantly.

Whisk in 15 ounces of pumpkin puree, 2 teaspoons of prepared yellow mustard, and 1/4 teaspoon of nutmeg. Stir continuously until well combined and heated through, about 2-3 minutes.

Gradually add 8 ounces of grated sharp cheddar cheese and 3/4 cup of grated parmesan cheese to the skillet, stirring until fully melted and the sauce is smooth and creamy, about 2-4 minutes.

Fold the cooked pasta into the cheese sauce, ensuring each shell is coated. If the sauce is too thick, add reserved pasta water a few tablespoons at a time until desired consistency is reached.

Season with salt and pepper to taste. Serve warm, topped with finely chopped chives.