Preheat oven to 400° F.
Place a large nonstick skillet over medium-high heat and heat oil. Add the onion and sauté for a few minutes.
Add ground meat into the skillet and cook for 8-10 minutes until meat is no longer pink, breaking apart with a wooden spoon or spatula while it cooks.
Add the marinara sauce. Cook 2-3 additional minutes.
Meanwhile cut the zucchini in half, lengthwise. Scoop the seeds and center out of each zucchini half, leaving about ¼ inch thick zucchini boats.
Place the zucchini in a 9X13 baking dish. Spoon the turkey mixture into the four zucchini boats. Sprinkle with cheese.
Add ½ cup water to the dish and cover with foil. Bake for 20-30 minutes until zucchinis are soft and cheese has melted.