Preheat the oven to 375°F. Cut the tops off the peppers and remove the seeds and membranes. Place the peppers upright in a baking dish.
Warm the vegetable broth in a saucepan over medium heat and keep it at a low simmer.
Heat the olive oil and butter in a large skillet over medium heat. Add the onion and garlic and cook until the onion softens.
Stir in the arborio rice and cook for two minutes until the grains look lightly toasted.
Add the pumpkin puree and chopped sage. Then ladle in the warm broth one cup at a time, stirring after each addition until the liquid absorbs before adding more.
Continue until the rice is creamy and tender, about 20 minutes. Season with salt and pepper, then stir in the parmesan cheese.
Spoon the risotto into the prepared peppers. Bake for 20 minutes until the peppers soften.