Preheat the oven to 375°F. Line two rimmed baking sheets with parchment paper or lightly coat them with cooking spray or oil, and arrange the squash rings on the sheets in a single layer.

Bring the quinoa and vegetable broth to a boil in a small saucepan. Reduce the heat to low, cover and simmer for about 15 minutes, until the liquid is absorbed. Fluff with a fork and let cool slightly.

Meanwhile, heat the olive oil in a medium skillet over medium heat. Add the onion and cook for about 10 minutes, until it is just beginning to brown. Add the apple and cook about 5 minutes more, until softened. Let cool slightly.

In a large bowl, combine the quinoa and the apple-onion mixture with the cheddar, cranberries, sage and walnuts. Season with salt and pepper to taste. Once the mixture is just warm, not hot, stir in the whisked egg.

In a small bowl, stir together the melted butter and maple syrup. Brush the tops and inside edges of the squash rings with the butter mixture and season with salt and pepper.

Spoon the quinoa filling into the center of each squash ring, pressing it down firmly to fill the ring without overflowing. Lightly spray the tops of the filling with oil or cooking spray. Bake for 30 to 40 minutes, until the tops are golden brown and the squash is tender when pierced with a fork. Use a thin spatula to transfer the rings to a serving platter.