Preheat the oven to 325 degrees F (165 degrees C).

Bring a large pot of water to a boil; add zucchini slices and cook until tender; drain well.

Heat olive oil in a medium skillet over medium heat; stir in onion and garlic and cook until onion is tender, about 2 minutes.

Combine zucchini, onion, and garlic in a 9x12-inch casserole dish; pour marinara sauce on top and stir well. Top with mozzarella cheese.

Bake in preheated oven until zucchini slices are heated through and cheese is bubbly, about 20 minutes.