Preheat the oven to 375°F (190°C).
Brush the cut sides of the acorn squash with olive oil and sprinkle with a pinch of sea salt. Place the squash cut-side down on a baking sheet lined with parchment paper. Roast in the preheated oven for about 30-40 minutes, or until tender.
While the squash is roasting, heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the chopped onion and cook until translucent, about 3-4 minutes.
Add the ground turkey and minced garlic to the skillet. Cook until the turkey is browned and cooked through, breaking it up with a spoon as it cooks.
Stir in the diced apple, dried sage, dried thyme, ground cinnamon, and ground nutmeg. Cook for an additional 5-7 minutes, until the apple is tender and the flavors are well combined. Season with sea salt and black pepper (if using).
Once the squash is done roasting, remove it from the oven and flip the halves cut-side up. Spoon the turkey mixture evenly into each squash half.
Return the stuffed squash to the oven and bake for an additional 10-15 minutes, or until the filling is heated through and the tops are slightly golden.
If desired, sprinkle with chopped walnuts for added crunch and nuttiness. Serve warm.