Place the halved gem squash on a baking sheet, cut side up. Drizzle with olive oil and season with salt and pepper. Roast for about 45 minutes or until tender.
While the squash is roasting, prepare the risotto. In a large pot, sauté the onion and garlic in olive oil until translucent.
Add the mushrooms and cook until they release their moisture and become tender.
Stir in the Arborio rice and cook for a few minutes until translucent around the edges.
Gradually add the vegetable broth and white wine (if using) while stirring continuously until the rice is cooked and creamy.
Stir in the peas and nutritional yeast (if using) until heated through. Season with salt and pepper.
Once the acorn squash is tender, remove from the oven. Fill each squash half with the mushroom and pea risotto.