01 - Preheat your oven to 400°F (200°C). In a bowl, toss the diced pumpkin with 1 tablespoon of olive oil and a pinch of salt. Spread the pumpkin in a single layer on a baking sheet and roast for 20 to 25 minutes, or until tender and lightly caramelized.
02 - While the pumpkin is roasting, bring a large pot of generously salted water to a rolling boil. Add the fettuccine or tagliatelle and cook according to package directions until al dente. Before draining, reserve approximately 1/2 cup of the pasta cooking water. Drain the pasta thoroughly.
03 - In a large skillet set over medium heat, warm the remaining 1 tablespoon of olive oil. Add the finely diced yellow onion and sauté for 3 to 4 minutes, until it becomes translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
04 - Add the roasted pumpkin to the skillet with the sautéed aromatics. Roughly mash the pumpkin with the back of a fork or a potato masher, aiming to leave some textural pieces intact.
05 - Reduce the skillet's heat to low. Add the unsalted butter and allow it to melt completely. Pour in the heavy cream and stir to combine it with the pumpkin mixture. Incorporate the freshly grated Parmesan cheese, ground nutmeg, salt, black pepper, and optional crushed red pepper flakes. Continue stirring until the cheese is fully melted and the sauce has thickened slightly, approximately 2 to 3 minutes.
06 - Add the drained, cooked pasta directly into the skillet with the Alfredo sauce. Toss gently to ensure all the pasta is evenly coated. If the sauce appears too thick, gradually add splashes of the reserved pasta water until a smooth, silky consistency is achieved.
07 - If using, stir in the baby spinach at this stage and continue tossing until it has just wilted into the dish.
08 - Serve the Fall Harvest Pumpkin Alfredo Pasta immediately. Garnish generously with extra grated Parmesan cheese and finely chopped fresh sage or parsley.