Heat a large skillet over medium high heat and add the oil. Once heated, add the chicken and onion. Saute, stirring often and breaking up the chunks of chicken, until the chicken is mostly cooked through, about 7 minutes. Add the garlic, Italian seasoning, salt, and pepper and cook for another 3 minutes or so, until the chicken is cooked through completely.
Meanwhile, place the zucchini slices on an oiled baking sheet. Season with a pinch of salt and pepper and bake for 5-8 minutes or until the slices are soft enough to roll without breaking.
Mix the ricotta with half of the shredded mozzarella. Lay out the zucchini slices and spread about a tablespoon of the ricotta mixture over the whole slice. Add a spoonful or so of the ground chicken on top of the ricotta on the zucchini. Roll from the bottom up, not too tightly, and place the roll-up in the baking dish. Repeat with the remaining zucchini slices.
Dollop the rest of the marinara sauce around and on top of the zucchini roll-ups. Top with the remaining shredded mozzarella cheese.
Increase the oven temperature to 425 degrees. Bake for 15-20 mins or until the sauce is bubbly and the cheese is melted and browned as desired. Serve hot with chopped parsley if desired.