1 ½ pounds Zucca di Mantova, halved crosswise and seeded
1 liter vegetable or chicken stock
6 tablespoons unsalted butter
1 small yellow onion, finely diced
6 fresh sage leaves, minced
1 ½ cups Arborio rice
1 cup dry white wine, at room temperature
¼ cup grated Parmesan cheese
Salt and freshly white ground pepper