Preheat your oven to 400 degrees F. Line a large rimmed baking sheet with parchment paper or foil for easy cleanup.

Brush the cut sides of the acorn squash halves with 1 tablespoon of olive oil and season them with 1/4 teaspoon of salt. Place them cut-side down on the prepared baking sheet. Roast for 25 minutes, until the flesh is fork-tender and the edges have started to brown. The squash should give easily when you press on the skin.

While the squash roasts, heat the remaining 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3 to 4 minutes, stirring occasionally, until it softens and turns slightly translucent.

Add the minced garlic to the skillet and cook for about 30 seconds, just until fragrant. Add the ground turkey, breaking it up with a wooden spoon as it cooks. Season with the remaining 3/4 teaspoon salt, black pepper, Italian seasoning, smoked paprika, and red pepper flakes if using. Cook for 6 to 8 minutes until the turkey is fully cooked through with no pink remaining.

Stir the marinara sauce into the turkey mixture and let it simmer together for 2 minutes. Remove the skillet from the heat.

Remove the baking sheet from the oven and carefully flip the squash halves cut-side up using tongs. Spoon the turkey filling evenly into each squash half, mounding it slightly. Sprinkle the shredded mozzarella over the top of each one.

Return the baking sheet to the oven and bake for an additional 10 minutes, until the cheese is melted and starting to bubble at the edges. Serve immediately, right in the squash shell.