Slice the yellow summer squash into roughly ¼-inch slices, dice the onion. Preheat a medium pan. Add oil or butter to the pan for cooking Add the yellow squash, onions, and garlic. Salt and pepper to taste. Cook about 5 to 7 minutes until slightly softened but still firm. Remove to a colander to drain excess liquid from squash mixture.
Combine in a bowl the mayonnaise, milk, and cheese. Salt and pepper to taste.
Grease your casserole dish. Add the squash and cheese mixture together into the casserole dish then mix well.
Combine the panko breadcrumbs, and additional Parmesan cheese. Top the squash casserole evenly with the topping.
Bake in a preheated 400-degree oven for thirty minutes until top is browned.
Serve immediately. May be stored for up to one week in an airtight container.