Cooking spray
3 tablespoons olive oil, divided
3 medium zucchini, sliced into rounds
1 onion, thinly sliced
1 garlic clove, minced
1 28-ounce can diced tomatoes with their juice
1 tablespoon Italian seasoning
½ teaspoon kosher salt
¼ teaspoon freshly ground black pepper
1 ½ cups panko breadcrumbs
½ cup freshly grated Parmesan cheese
¾ cup shredded mozzarella cheese
Preheat the oven to 350 degrees Fahrenheit and spray a 13-by-9-inch baking dish with cooking spray.
In a large skillet, heat 1 tablespoon of the oil over medium-high heat. Add the zucchini and cook, stirring occasionally, until tender, about 5 minutes. Remove the zucchini from the skillet using a slotted spoon and set aside in a bowl.
Heat the remaining 2 tablespoons of oil in the same skillet over medium-high heat. Add the onion and garlic. Cook, stirring occasionally, for 1 minute. Stir in the tomatoes, Italian seasoning, salt and pepper, and let simmer, uncovered, for 10 minutes. Remove the skillet from the heat.
Stir the reserved zucchini into the mixture in the skillet, and then transfer the whole mixture to the prepared baking dish. Sprinkle the panko and Parmesan cheese over the top. Cover with aluminum foil.
Bake for 20 minutes.
Remove the foil cover and sprinkle the mozzarella cheese over the top. Return to the oven and bake, uncovered, for 10 minutes more, until the cheese is melted and bubbling.
Serve hot.