Sauté the veggies: Melt half of the butter in a large skillet over medium heat. Add the chopped onion and sliced squash, stirring gently until they soften and become slightly translucent. Season with salt, pepper, and a pinch of garlic powder if you like.

Drain the moisture: Once tender, transfer the mixture to a colander or sieve. Let it drain thoroughly so the casserole won’t become watery. I like to gently press on it to release excess liquid.

Prepare the creamy base: In a separate bowl, melt the rest of the butter. Whisk in the beaten eggs, milk, sour cream, and half of the shredded cheese until smooth and well combined.

Combine: Return the slightly drained squash and onions to the bowl and fold them gently into the creamy mixture. Make sure everything is evenly coated.

Build the casserole: Pour the mixture into a greased baking dish. Sprinkle the remaining cheese over the top. Then combine the crushed crackers with a little melted butter, and scatter this mixture evenly on top to create a crunchy crust.

Bake: Place the casserole in a 350°F (175°C) oven and bake for about 25–30 minutes, or until the top is golden brown and bubbly.

Rest and serve: Let the casserole rest a few minutes before serving — this helps it set nicely and makes it easier to scoop.