Prepare the pasta according to the package instructions, cooking until al dente. Drain and set aside, keeping it warm while preparing the sauce.
Heat a large skillet over medium heat and drizzle in the olive oil. Once the oil is warm, add the minced garlic and sauté briefly until fragrant, being careful not to burn it. Whisk in the tomato paste, pumpkin puree, and half and half, stirring until the mixture is velvety smooth. Gradually pour in the vegetable broth, followed by the nutmeg, salt, and black pepper. Stir well and allow the sauce to gently simmer for about 5 minutes, letting the flavors meld.
Toss in the baby spinach, stirring until it begins to wilt slightly. Add the cooked pasta to the skillet, folding it into the sauce to ensure each piece is evenly coated.
Sprinkle the dish with toasted walnuts and, if desired, a generous grating of Parmesan cheese. Serve immediately while warm, enjoying the comforting combination of creamy pumpkin, aromatic spices, and crunchy toppings.