In a large bowl, combine pumpkin puree, flour, egg, salt, nutmeg, and Parmesan cheese (if using). Mix until a dough forms.

Turn the dough out onto a lightly floured surface and knead for 2-3 minutes, adding more flour as needed to prevent sticking. The dough should be soft but not too sticky.

Roll the dough into a long rope, about 1 inch thick. Cut the rope into 1-inch pieces.

Using a fork, gently roll each gnocchi piece over the tines to create ridges. This helps the sauce cling to the gnocchi. Lightly flour a baking sheet and place the gnocchi on it.

While the gnocchi are resting, make the sauce: In a large skillet, melt butter over medium heat. Cook until butter turns light brown and nutty, about 3-5 minutes. Add minced garlic and cook for 1 minute, until fragrant.

Stir in diced tomatoes (undrained) and bring to a simmer. Reduce heat to low and simmer for 10 minutes, allowing the sauce to thicken slightly. Stir in heavy cream and Parmesan cheese. Season with salt and pepper to taste. Simmer for another 5 minutes, or until desired consistency is reached.

Bring a large pot of salted water to a boil. Gently add the gnocchi to the boiling water and cook for 2-3 minutes, or until they float to the surface.

Remove the gnocchi with a slotted spoon and add them to the sauce. Toss to coat the gnocchi in the sauce. Serve immediately, garnished with fresh sage leaves (if desired).