Preheat the oven to 400F.
Line a baking sheet with parchment paper. Mix 1 tbsp olive oil and 1 tbsp maple syrup in a small bowl and brush the acorn squash halves with the mixture. Place the squash halves cut side down and roast until fork-tender (about 40 mins).
In the meantime, add the quinoa and water to a medium saucepan and cook until the water has been absorbed (about 15 mins). Remove from the heat and stir in the dried cranberries and mulberries. Cover and set aside for the dried berries to plump up and for the quinoa to cool a bit.
Toast the pumpkin seeds in a toaster oven or in a small pan for a few minutes until lightly browned. Set aside.
In a large bowl, add the quinoa, onions, parsley celery, pumpkin seeds and feta cheese. Mix well and dress with lemon juice, the remaining 1 tbsp olive oil and 1 tbsp maple syrup, salt and pepper.
When the squash is ready, remove from the oven. Flip and fill each one with the quinoa mixture.