In a large skillet, over medium heat, add the olive oil and sauté the onion until it is tender.
Add the garlic and sausage and cook it until the sausage is no longer pink. Add the tomato sauce and salt and pepper to taste. Reduce the heat to low and simmer sauce for 15 minutes or longer.
Ladle sauce to cover the bottom of a 9 x 13 pan. Layer the sliced zucchini over the sauce overlapping the edges slightly. Layer the cheese mixture, and follow with a layer meat sauce.
Repeat layering until the pan is full. Top with remaining mozzarella cheese and sprinkle with the reserved parsley.