Slice the summer squash thinly using a sharp knife or mandoline, then sprinkle with a little salt and let sit for 10 minutes to draw out excess moisture. Pat dry with a paper towel to keep the casserole from becoming watery.
Preheat your oven to 375°F (190°C). In a small bowl, combine the panko breadcrumbs with melted butter and dried herbs, then set aside for topping.
In a skillet, sauté diced onion until translucent and fragrant, about 5 minutes. Remove from heat and let cool slightly.
In a mixing bowl, whisk together the eggs, heavy cream, chopped herbs, salt, and pepper to make a creamy custard base.
Layer the sliced squash evenly in a greased baking dish, slightly overlapping, then sprinkle the sautéed onion and shredded cheese over the top.
Pour the custard mixture evenly over the layered squash, ensuring all slices are coated and the filling begins to set.
Sprinkle the breadcrumb mixture over the top of the casserole, covering it evenly for a crispy finish.
Bake in the preheated oven for 40 minutes, or until the top is golden brown and bubbling around the edges. Cover loosely with foil if it browns too quickly.
Remove from the oven and let rest for 5 minutes; this helps the custard set further and makes serving easier.
Slice into squares and serve warm, enjoying the tender squash contrasted with the crispy topping and creamy filling.
