Preheat oven to 375 degrees F. Spray a small baking dish with cooking spray and set aside (I used a dish that was 8-in x 5-in, but an 8-inch square pan will also work).
Place zucchini in a colander, sprinkle with salt, and allow to sit for about 10 minutes (this will help draw out some of the water from the zucchini). After 10 minutes, squeeze out moisture from the zucchini.
In the prepared dish, combine zucchini, onion, garlic, eggs, ½ cup Parmesan cheese, mozzarella cheese, and cheddar cheese. I like to use my hands to make sure that it’s all completely combined, and then gently press the mixture evenly into the prepared dish.
Bake uncovered for 20 minutes.
Meanwhile, in a small bowl, pour melted butter over breadcrumbs and 2 tablespoons of Parmesan cheese. Stir until completely combined.
After 20 minutes, remove zucchini from the oven. Sprinkle buttered breadcrumbs over the zucchini and return the dish to the oven. Continue baking for about 5-10 more minutes, or until the top is golden brown and crispy.
Traditional gratins rely on heavy cream for structure, but this vegetable bake depends entirely on eggs and cheese binding agents. Most squash bakes turn into soup because cooks skip the critical pressing phase. Zucchini cells hold vast amounts of water that must be physically forced out before baking begins. Salting the grated squash in a colander draws out internal moisture via osmosis during a ten minute rest. Squeezing that liquid out by hand transforms the final texture entirely. Without this step, the mozzarella and melted butter simply float on a pool of vegetable steam.
Managing Squash Moisture
Place the grated squash into a mesh colander over the sink and apply coarse salt evenly. Let gravity do initial work for ten minutes. Gather the drained vegetables inside a clean dish towel and twist tightly to expel every remaining drop. Skipping the towel extraction leaves too much liquid behind, preventing the cheeses from setting properly into a firm sliceable wedge.
Browning the Panko Layer
Coat the panko flakes in melted butter and aged parmesan before applying them halfway through the bake. This prevents the breadcrumbs from burning during the initial twenty minutes at 375 degrees Fahrenheit. Uncovered baking lets excess steam escape before the final ten-minute browning window creates a crisp crust.