Preheat your oven to 400°F (200°C).
Slice the acorn squash in half lengthwise and scoop out the seeds. Brush the cut sides with olive oil and season with salt and pepper. Place them cut-side down on a baking sheet lined with parchment paper.
Bake the squash in the preheated oven for about 25-30 minutes, or until tender when pierced with a fork.
While the squash bakes, heat the olive oil in a skillet over medium heat. Add the chopped onion and sauté for about 3-4 minutes until translucent.
Add the chopped kale, apples, thyme, and sage to the skillet. Cook for another 5-7 minutes until the kale is wilted and the apples are tender. Season with salt and pepper.
In a large mixing bowl, combine the cooked quinoa, sautéed mixture, and toasted walnuts. Mix well to combine all ingredients.
Carefully remove the acorn squash from the oven and turn them cut-side up. Fill each half generously with the quinoa mixture.
If using cheese, sprinkle it over the top of the stuffed squash.
Return the stuffed squash to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool slightly before serving.