Creating Stuffed Butternut Squash Gratin is a straightforward process. Just follow these simple steps for a delicious result:

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Prepare the Squash: Cut the butternut squashes in half lengthwise and scoop out the seeds. Place them cut-side up on a baking sheet.
3. Roast the Squash: Drizzle with olive oil, sprinkle with salt, and roast in the preheated oven for about 25-30 minutes, or until tender.
4. Cook the Quinoa/Rice: While the squash is roasting, rinse the quinoa or rice. In a pot, combine it with vegetable broth or water and bring to a boil. Reduce heat, cover, and simmer until cooked (about 15 minutes for quinoa, 20 for rice).
5. Sauté the Vegetables: In a frying pan, heat a bit of olive oil over medium heat. Add the chopped onion and diced mushrooms, cooking until soft. Stir in the chopped spinach and cook until wilted. Season with garlic powder, thyme, salt, and pepper.
6. Combine Filling Ingredients: In a large bowl, mix the cooked quinoa/rice, sautéed vegetables, grated cheese, and breadcrumbs. Adjust seasoning as necessary.
7. Stuff the Squash: Once the squash is done roasting, fill each half with the stuffing mixture, pressing down gently to pack it in.
8. Add Cream (Optional): For a creamier texture, drizzle cream or milk over the stuffed squash.
9. Bake Again: Return the stuffed squash to the oven and bake for an additional 15-20 minutes, until the top is golden and crispy.
10. Serve: Remove from the oven and let cool for a few minutes before serving.

These straightforward steps will guide you to create a delicious and visually stunning Stuffed Butternut Squash Gratin!