Place the yellow squash on a large cutting board, trim off the ends with a chef’s knife, then slice the squash into ¼-inch rounds.

Chop the onion on the cutting board with the chef’s knife into small pieces.

Set a large skillet on the stove over medium heat, add 3 tbsp butter, and stir with a wooden spoon until the butter melts.

Add the sliced squash and chopped onion to the skillet, sprinkle 1 tsp salt and ½ tsp pepper over the top with measuring spoons, and cook for 10 minutes, stirring often with the wooden spoon until the squash softens and some of the liquid cooks off.

Turn off the heat and let the squash mixture cool slightly in the skillet.

Crack 2 eggs into a large mixing bowl and beat them with a whisk until smooth.

Add 1 cup shredded cheddar, ½ cup sour cream, and ¼ cup milk to the bowl and whisk until the mixture looks creamy and combined.

Spoon the warm squash and onion mixture from the skillet into the mixing bowl and stir with the wooden spoon until all the slices are coated in the cheesy mixture.

Put ¾ cup breadcrumbs or crushed crackers into a small bowl and pour 3 tbsp melted butter over them with a measuring spoon.

Stir the crumbs and butter together with a spoon until the crumbs look evenly moistened.

Lightly grease a 9×13 inch baking dish, then pour the squash mixture into the dish and spread it into an even layer with the wooden spoon.

Sprinkle the buttery crumb mixture evenly over the top of the squash in the baking dish.

Place the baking dish on the middle rack of a preheated 350°F oven using oven mitts and bake for 35 minutes until the casserole is bubbling around the edges and the topping is golden brown.

Remove the baking dish from the oven with oven mitts and let the casserole sit for 5 minutes so it sets slightly, then scoop portions out with a large spoon and serve warm as a side dish.