Heat oven to 190 C. Lightly coat an 20 by 20 cm baking dish with cooking spray.
Warm a large skillet over medium heat. Add olive oil then onion and a pinch of salt. Cook until soft.
Add zucchini and cook until edges look glossy and the pan holds very little liquid.
Stir in garlic basil and green onions. Cook one minute then take off the heat.
Fold in half of the mozzarella and half of the parmesan.
Spoon the mixture into the dish. Top with the rest of the cheeses. Sprinkle panko if using.
Bake until the top is deep golden and the center feels tender about 25 to 30 minutes.
Rest five minutes so the cheese settles. Scoop and serve warm.