Toast the pepitas on a small sheet tray in a 350°F oven for 6-7 minutes or toast in a small skillet over moderate heat.
Preheat oven to 425°F.
Peel pumpkin and spray with cooking spray or drizzle with olive oil; season with salt. Roast to just tender but not brown, about 25 minutes; cool.
Shred pumpkin with the large side of a box grater or shred it with tines of 2 forks.
Bring a large pot of water to a boil for pasta.
Heat a large skillet over medium-high heat with EVOO, 2 turns of the pan. Add onions, garlic, salt, pepper and nutmeg, and sauté 5 minutes. Add stock and rosemary, and reduce heat to a low simmer. Stir in grated pumpkin and simmer 15 minutes.
In a small skillet, melt butter over medium heat. Add sage and crisp leaves; remove to cool on a paper towel. Pour brown butter into pumpkin sauce and wilt in the shredded kale.
Salt boiling pasta water and drop pasta, cook to 1 minute less than suggested on package for al dente, reserving 1 cup water before draining. Add starchy water to pumpkin sauce.
Toss drained pasta with pumpkin sauce and a few handfuls of grated cheese. Garnish with pepitas and crispy sage leaves.