Preheat oven to 425 degrees. Line large baking sheet with parchment paper (for easy cleanup) or lightly spray with non-stick spray; set aside.
Place pumpkin cubes in bowl and toss with 2-3 tablespoons melted butter. Spread on prepared baking sheet; salt and pepper to taste. Roast 30-35 minutes or until pumpkin is tender. Use approximately 2-cups roasted pumpkin in the recipe; use remaining roasted squash as a side dish.
Saute chopped onion in 2-tablespoons butter, over medium heat in a large saucepan or skillet, until translucent and just beginning to caramelize. Add garlic and stir until fragrant. Add rice and saute 1-2 minutes or until rice grains are coated. Stir approximately ½-cup chicken broth into rice; stirring frequently, cook until rice has absorbed broth. Add ground nutmeg. Continue to stir in simmering broth, ½-cup at a time, stirring frequently, allowing each addition of broth to be absorbed by rice before adding more; cook until rice is tender (about 20 minutes).
Stir in hot roasted pumpkin cubes during last 2-3 additions of broth. Stir in shredded parmesan cheese and additional broth to create desired consistency. Garnish with toasted chopped walnuts, additional parmesan cheese and a sprinkle of ground nutmeg.