Slice the summer squash thinly and place them in a bowl. Squeeze out excess moisture with your hands to prevent sogginess, then set aside.
In a skillet, melt a bit of butter and sauté the chopped onion and minced garlic until fragrant and slightly softened, about 2 minutes. This fills your kitchen with a warm aroma and softens the onion for easy mixing.
In a large mixing bowl, whisk together the beaten eggs, milk, chopped herbs, salt, and pepper. This creates a creamy, flavorful binder for the casserole.
Add the sautéed onion and garlic to the egg mixture, then fold in the sliced squash. Mix gently until everything is evenly coated and integrated.
Gradually add the shredded cheddar cheese, stirring gently to distribute it evenly without clumping, ensuring a cheesy, flavorful layer inside.
Pour the mixture into a greased baking dish, spreading it out evenly. You should see the slices layered in a slightly mounded shape.
Sprinkle the toasted breadcrumbs evenly over the top, then wait a moment for the topping to settle into a golden, crispy crust during baking.
Bake the casserole in a preheated oven at 375°F (190°C) for about 40 minutes, watching for bubbling and golden brown edges. Cover loosely if it browns too fast to keep the top crispy but not burnt.
Once baked, remove the casserole from the oven and let it rest for a few minutes. The bubbling will subside slightly, making it easier to serve clean slices.
Slice into portions and serve warm, enjoying the tender squash with a crispy topping and cheesy, savory flavor in every bite.
