Begin by bringing a large pot of salted water to a rolling boil over high heat. Add the fettuccine and cook until al dente, about 8-10 minutes. Drain and set aside, reserving a bit of the pasta water.

In a large skillet, melt the salted butter over medium heat. Add fresh sage leaves and cook for about 4 minutes until the butter turns golden brown and the sage becomes crispy. Remove the sage leaves and set aside.

Reduce heat to low and add chopped sage, minced garlic, and chili flakes to the browned butter. Sauté gently for about 1 minute until fragrant.

Incorporate the pumpkin puree and apple cider into the skillet. Stir well and let simmer for 2-3 minutes until slightly thickened.

Slowly whisk in the milk and increase heat to a gentle simmer. Allow sauce to thicken for about 5 minutes, stirring occasionally. Add Parmesan and Gouda cheese until melted, then season with nutmeg, salt, and pepper.

Add the cooked pasta to the skillet, tossing to coat in the sauce. Let simmer for an additional 3-5 minutes, adding reserved pasta water if the sauce is too thick.

Serve immediately, topping with crispy sage leaves for added crunch and flavor.