Heat oven to 375°F. Rub a rimmed baking sheet with oil and place pumpkin, cut-side down, on the sheet. Roast 60 minutes, until pumpkin flesh is tender when pierced with a fork. Remove from oven. When pumpkin is cool enough to handle, scrape flesh from the skin. Discard skin and set flesh aside for at least 30 minutes to cool.

While the pumpkin is roasting, cook the fettuccine according to package directions. Drain and set aside.

Once cool, divide pumpkin in half. Cut one half into ½-inch cubes and set aside. Purée remainder in a blender or food processor until smooth, adding stock as needed to loosen.

Slice 16 sage leaves into thin strips. Line a plate with paper towels.

Coat bottom of a large pan with oil and set over medium-high heat. Fry sliced sage leaves until crisp, 1 to 2 minutes. Remove from pan and let drain on paper towels. Add remaining 4 whole sage leaves and fry until crisp, turning once.

Drain oil from pan and wipe with a paper towel. Reduce heat to medium and return pan to stovetop. Add butter and cook until foaming subsides. Add onion and garlic and cook, stirring, until translucent and soft, 2 to 3 minutes. Add pumpkin purée and stir to combine. Add cream, season with salt and pepper, reduce heat to medium-low and cook 15 minutes, until mixture thickens slightly. Add the pumpkin cubes and stir to combine.

Add cooked fettuccine to the pumpkin mixture along with the cheese, and stir to combine. Add fried sage strips and toasted pepitas and adjust seasoning. Divide fettuccine between four shallow bowls and top each with whole fried sage leaf.