Heat the oven to 400°F. Brush the cut sides of the spaghetti squash with olive oil, then season with ½ teaspoon salt and ¼ teaspoon black pepper. Place the halves cut side down on a baking sheet.

Roast the squash until the flesh is tender when pierced with a fork but the shells still hold their shape, about 35 to 45 minutes. Let it cool enough to handle. Use a fork to loosen the strands, leaving them inside the shells.

While the squash roasts, combine the ground turkey, chopped spinach, panko, egg, ¼ cup Parmesan, Italian seasoning, remaining ½ teaspoon salt, and remaining ¼ teaspoon black pepper in a bowl. Mix gently until combined.

Divide the turkey mixture among the four squash halves, shaping it into an even layer over the loosened squash. Spoon the marinara sauce over the filling and sprinkle with the remaining ¼ cup Parmesan.

Return the filled squash halves to the oven. Bake until the turkey mixture is cooked through and reaches 165°F in the center, about 20 to 25 minutes. Let the boats rest for a few minutes before serving.