Preheat oven to 350 degrees. Lightly oil the bottom of a 12″ Versa Skillet or a 9×13 casserole dish.
Heat the oil in a large skillet over medium heat; add the garlic and onion and saute for 2 to 3 minutes.
Add the spinach and saute until limp, about 2-3 minutes.
Add the ground turkey, if using.
Season with salt to taste. Stir in the basil and the cooked spaghetti squash flesh and gently toss the ingredients together.
Add the cooked spaghetti and gently toss the ingredients once more. Remove skillet from heat and set aside.
In a medium bowl, whisk together the marinara sauce and the egg. Stir the sauce into the spaghetti mixture and gently toss to coat. Transfer the spaghetti squash mixture to the prepared casserole.
Bake, uncovered, for 20 to 25 minutes, or until firm. Remove from oven and allow to cool 10 minutes before serving. Serve warm.