To roast the squash: Heat oven to 350°. Cut 1 squash in half lengthwise; remove and discard seeds. Place 1 tablespoon butter, 1 sprig rosemary and 1 sprig sage into the cavity of each half. Season flesh with salt and pepper, then wrap halves individually in aluminum foil. Put squash on a baking sheet, cavity-side up, and roast until very soft, about 2 hours.

Remove squash from oven and carefully unwrap; discard herbs. Scoop flesh from skins; discard skins. Purée flesh in a food processor until smooth (squash purée can be made up to 2 days ahead).

Peel and seed remaining squash; cut flesh into 1/8-inch dice.

 

To make the risotto: Combine 3 tablespoons of the remaining butter with oil in a large heavy pot and heat over medium heat until butter is melted. Add onion, season with salt and pepper and cook, stirring occasionally, until onion begins to soften, about 5 minutes. Add rice and increase heat to medium-high. Using a wooden spoon, stir until rice is opaque and begins to pop, 2 to 3 minutes. Add wine; cook, stirring, until wine is mostly evaporated, about 1 minute. Add enough broth to just cover the rice, about 2 cups. Actively simmer the broth, stirring and scraping rice away from sides of pot. Cook until broth is mostly evaporated, about 5 minutes.

Add the diced (uncooked) squash and enough broth to just cover the rice mixture. Reduce to a gentle simmer and cook, stirring occasionally, until broth is mostly evaporated, about 5 minutes more. Add 1/2 cup broth; cook, stirring frequently, until broth is mostly evaporated. Continue cooking in this manner, adding broth in 1/2 cupfuls until the rice is just barely tender and all’onda (a bit liquid); you may have broth leftover.

 

To fry the sage: Heat oil in a small heavy skillet over medium-high heat until it shimmers, then fry sage leaves in 3 batches, stirring, until crisp, 30 seconds to 1 minute per batch. Using a slotted spoon, transfer to paper towels to drain.

To serve: Stir puréed squash and mostarda into risotto, then add cheese and remaining 3 tablespoons butter; stir to combine. Thin with a little more broth, if desired. Season with salt and pepper. Serve immediately, garnished with amaretti, cheese and fried sage.