In a small nonstick skillet, melt 2 tablespoons of the butter over medium heat. Add 1 grated garlic clove and cook until fragrant, about 30 seconds. Stir in the panko breadcrumbs, ½ teaspoon salt, and ¼ teaspoon pepper and toast, stirring frequently, until the panko is golden brown, 2 minutes.

Remove the mixture to a small bowl and cool for 5 minutes. Stir in the parmesan.

In a large pot over medium-high heat, bring water to a boil. Add the pasta and salt to taste. Cook, stirring the pasta, until just under al dente, 5 to 6 minutes. Drain the pasta, reserving ½ cup of the pasta water.

In the same pot over medium heat, melt the remaining ¼ cup of butter. Stir in the sage leaves. Cook, stirring frequently, until the sage is lightly fried and the butter is deeply golden and smells toasty. Remove the sage leaves to a paper towel-lined plate and add the onion to the butter. Cook until the onion is soft and translucent, 3 to 4 minutes. Add the remaining 2 grated garlic cloves and cook until fragrant, about 30 seconds. Whisk in the flour, mustard, nutmeg, cayenne, the remaining 1 ½ teaspoons of salt, and the remaining ½ teaspoon pepper. Cook until the raw flour smell disappears, 3 to 4 minutes. Gradually whisk in the milk until smooth. Whisk in the pumpkin puree and cook until very thick, 5 minutes.

Remove the pot from heat and gradually add the shredded cheeses, whisking until melted before adding more. Taste for salt.

Fold in the drained noodles, adding pasta water 1 tablespoon at a time until the sauce coats the noodles.

Serve the pumpkin mac and cheese hot, sprinkled with the toasted panko bread crumbs and crumbled sage.