In a medium saucepan, bring broth to a boil and then reduce to a low simmer. Keep heat on low.
Meanwhile, in another medium saucepan, melt the butter over medium heat. Add onion and cook, stirring occasionally, until softened, about 5 minutes. Add garlic and sage and cook, stirring, until fragrant, about 1 minute.
Add the rice, salt, and nutmeg. Cook, stirring constantly, until the rice is coated with the onions mixture and lightly toasted, about 3 minutes.
Use a ladle to add ½ cup of broth, stir, then allow the broth to be almost all absorbed, 2-3 minutes. Keep adding more broth, ½ a cup at a time, and allow the broth to get absorbed undisturbed, until all the broth is incorporated, about 20 minutes total.
Remove the saucepan from heat. Stir in the pumpkin puree, Parmesan cheese, and red wine vinegar, and serve immediately.