Preheat the oven to 400°F and line a baking sheet with parchment paper.
Cut the squash in half lengthwise. Lightly coat the cut sides with oil and season with salt and black pepper.
Place cut-side down on the baking sheet and roast for 45 minutes, until fork-tender.
Heat 1 tablespoon olive oil in a skillet over medium heat. Add the onions and cook for 5 to 10 minutes, until softened and golden.
Add the ground turkey and cook until no longer pink, breaking it into small pieces as it cooks.
Remove from the heat and stir in the quinoa, sage, oregano, garlic powder, white pepper, and tamari. Season with salt if needed.
Turn the squash cut-side up and scoop out the seeds. If using butternut squash, scoop out some flesh from the neck and stir it into the filling.
Spoon the turkey filling into the squash and bake for another 15 to 20 minutes.
Add the Dijon mustard, maple syrup, red wine vinegar, extra virgin olive oil, garlic, salt, and black pepper to a jar. Shake until emulsified.
Heat 1 tablespoon olive oil in a small skillet over medium heat. Fry the sage leaves for about 30 seconds, until crisp, then drain on paper towels.
Drizzle the dressing over the stuffed squash and garnish with walnuts and crispy sage leaves.