Preheat the oven to 400°F (200°C) and line a rimmed baking sheet with parchment paper.
Brush the flesh side of the acorn squash halves with 1 tablespoon of olive oil and season lightly with salt and pepper.
Place the squash cut-side down on the prepared baking sheet and roast for 30 to 35 minutes, or until the flesh is fork-tender.
While the squash roasts, heat the remaining 1 tablespoon of olive oil in a large skillet over medium-high heat.
Add the ground turkey or sausage to the skillet, breaking it into small crumbles with a spatula, and cook until browned and fully cooked through.
Stir in the diced onion and apple, cooking for 5 minutes until the onion is translucent and the apple begins to soften.
Add the minced garlic, dried sage, thyme, salt, and pepper; sauté for 1 minute until fragrant.
Add the chopped baby spinach to the skillet and stir until just wilted, then remove the filling from the heat.
Remove the roasted squash from the oven and carefully flip them over to be cut-side up.
Divide the protein filling evenly among the four squash cavities, mashing the squash flesh slightly with a fork before stuffing if more space is needed.
Return the stuffed boats to the oven for 10 minutes to allow the flavors to meld and the filling to heat through.
Top with toasted walnuts before serving.