Heat the oven to 425F (220C) and line a large baking sheet with parchment. Toss the cubed squash with 2 tablespoons of the olive oil, the salt, and the pepper, then spread it in a single layer. Roast for 25 to 30 minutes, tossing once halfway, until the cubes are tender and browned at the edges.

Measure out about 1 1/2 cups of the roasted squash into a bowl. Mash it with a fork, or blend it with 1/4 cup of the warm vegetable stock, until you have a mostly smooth puree. Set the puree aside and keep the remaining squash cubes warm.

Heat the remaining 4 cups or so of vegetable stock in a saucepan until it just simmers, then drop the heat to low and keep it covered. Hot stock keeps the risotto cooking evenly and is the difference between a creamy pot and a gluey one.

In a wide, heavy pot or Dutch oven, melt 2 tablespoons of the vegan butter with the remaining 1 tablespoon olive oil over medium heat. Add the onion and cook for 6 to 8 minutes, until soft and translucent. Add the garlic and cook for 1 minute more, until fragrant.

Add the arborio rice and stir for 1 to 2 minutes, until the grains are coated and the edges turn slightly translucent. Pour in the white wine and stir until it has almost fully absorbed.

Add the hot stock one ladle at a time, stirring every 30 seconds or so and waiting until each ladle is nearly absorbed before adding the next. After about 15 minutes, stir in the squash puree and half of the sage. Keep adding stock and stirring until the rice is tender with a slight bite in the center, 25 to 30 minutes total.

Take the pot off the heat. Stir in the nutritional yeast, the remaining 1 tablespoon vegan butter, and the rest of the sage. Fold in the reserved squash cubes, taste, and add salt if it needs it. Let the risotto rest for 2 minutes, then loosen it with a splash of warm stock if it has tightened up. Serve in warm bowls.