Preheat your oven to 375°F (190°C).
- Cut the acorn squashes in half and scoop out the seeds. Place them cut-side down on a baking sheet lined with parchment paper. Roast for 35–45 minutes, or until the squash is tender and easily pierced with a fork.
- While the squash is roasting, prepare the filling. Heat olive oil in a skillet over medium heat. Add the chopped onion and garlic, and sauté until softened, about 5 minutes.
- Add the cooked quinoa (or your choice of grains), lentils, spinach (if using), cranberries, and chopped nuts to the skillet. Stir to combine. Season with cinnamon, cumin, salt, and pepper, and cook for an additional 3–5 minutes to heat through.
- Once the squash is done roasting, remove it from the oven and flip the halves over. Stuff each squash half with the prepared filling, pressing gently to pack it in.
- Return the stuffed squash to the oven and bake for an additional 10–15 minutes to allow the flavors to meld and the filling to warm through.
- Garnish with fresh herbs before serving.