Preheat oven to 400 degrees F.
Cut both squash in half lengthwise and scoop out the flesh, leaving 1/2 inch of flesh on the skin, and place on a greased baking sheet rind side down. Save the flesh for another use.
Heat coconut oil in a skillet over medium high heat.
Add onions, salt, and pepper, and saute until soft.
Add ground beef, salt, and pepper, and stir until almost cooked through.
Add garlic, marinara sauce, oregano, basil, and thyme, and cook while stirring for about 5 minutes.
Sprinkle in arrowroot starch and stir until the sauce thickens.
Fill the ground beef mixture into the squash.
Top with cheese if using, and sliced olives.
Bake for 20-25 minutes until everything is cooked through and cheese is melted and bubbling.