Preheat the oven to 375°F and line a baking sheet with parchment paper.
We’ll start by roasting the squash. Wash, slice off the tops, and scoop the seeds and stringy flesh from the centre. Season the insides with olive oil, salt, and pepper. Place the prepared squash on a baking sheet, cut side down, and roast in the oven until they become tender but still hold their shape, about 35 minutes.
While the squash is roasting, we’ll start the filling. In a large saucepan, bring the vegetable broth to a boil. Add the wild rice and quinoa, reduce heat to low, cover, and simmer for 35 to 40 minutes, or until both grains are cooked and the liquid is absorbed. Remove from heat and let it sit, covered, for 10 minutes—fluff with a fork.
While the rice and quinoa sit, we’ll finish the rest of the filling. Heat olive oil in a medium-sized skillet over medium-high heat and add the chopped mushrooms. Season with salt and pepper and sauté until they become golden brown, about four to five minutes. Remove from heat.
Combine the spinach, cranberries, walnuts, mascarpone, Parmesan, garlic, cinnamon and nutmeg in a mixing bowl. Mix until all the ingredients are well combined. Add the grain mixture and the sautéed mushroom and mix again.
Once the acorn squash is tender, remove it from the oven, flip it over, and cut it up. Stuff each squash cavity with the filling, pressing it down gently to pack it in.
Return the stuffed squash to the oven and bake for 15 to 20 minutes or until the edges are slightly browned, the filling is heated, and the skin can easily be cut. Remove from the oven and let cool slightly before serving. Enjoy!