Heat a tablespoon of olive oil in a skillet over medium heat. Sauté the onion, garlic, and sausage until the sausage is browned, 3-4 minutes. Add the Italian seasoning, marinara sauce, and a pinch of salt and pepper. Cook until the excess liquid has evaporated, about 8 minutes.
Use a sharp knife to trim the ends from the zucchini, then slice in half lengthwise. Use a teaspoon and run it along the center of the zucchini to scoop out all the seeds and make a "boat."
Place the zucchini on a baking sheet and brush with olive oil. Fill the zucchini with the sausage mixture, then sprinkle with the cheese. Bake until the cheese is melted and golden brown, 20-25 minutes.