Preheat the oven to 375°F. Cut the spaghetti squash in half lengthwise and scoop out seeds. Place cut side down on a baking sheet and bake for 45 minutes.

While the squash is baking, season chicken breasts with salt, garlic powder, and oregano, then grill until cooked through. Let rest and slice thinly.

Once the squash is cooked and cooled slightly, fluff the strands with a fork and transfer to a bowl. Mix in marinara sauce and half of the mozzarella cheese.

Spoon the mixture back into the squash shells, top with sliced grilled chicken and remaining mozzarella cheese.

Bake the stuffed squash halves in the oven for an additional 15 minutes until cheese is golden and bubbly.

Remove from oven, garnish with fresh basil if desired, and serve hot.