Preheat the oven to 375°F. Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers cut-side up in a baking dish.
Warm the vegetable broth in a saucepan over low heat and keep it nearby.
Heat the olive oil in a large skillet over medium heat. Add the onion and cook until softened, about 4 minutes. Stir in the garlic and cook for 1 minute more.
Add the arborio rice to the skillet and stir for 2 minutes until the grains look lightly toasted. Ladle in 1 cup of warm broth and stir until mostly absorbed. Continue adding broth 1/2 cup at a time, stirring often, until the rice is tender and creamy, about 18 minutes total.
Stir the pumpkin puree, chopped sage, and half the Parmesan into the risotto. Season with salt and black pepper. Remove from heat.
Spoon the risotto evenly into the prepared peppers. Sprinkle the remaining Parmesan over the tops. Bake for 20 minutes until the peppers are tender and the tops are lightly browned.