Preheat oven to 450 degrees. In a large skillet, melt the butter over medium heat; add zucchini, onion and garlic. Cook, stirring occasionally until vegetables are tender, about 6 to 8 minutes.
Add the half-and-half and cream cheese to the vegetable mixture; stir to combine, allowing the cream cheese to melt.
Set aside 4 tablespoons of the panko and 5 tablespoons of the Parmesan cheese. Add the remaining 6 tablespoons of the panko and 5 tablespoons of Parmesan cheese, salt and pepper to the vegetable mixture; stir to combine.
Coat a 2-quart shallow baking dish with nonstick spray. Spoon mixture into the baking dish. Sprinkle the top with remaining panko and Parmesan cheese.
Bake until top is golden brown, about 10 to 12 minutes.