Preheat your oven to 325°F (165°C). Grease a 1½-quart casserole dish lightly with butter or cooking spray.

In a large pot, bring water to a boil and add the sliced yellow squash, zucchini, and diced onion. Cook for about 15 minutes or until tender. Drain thoroughly.

In a large mixing bowl, combine the drained vegetables, shredded cheddar cheese, biscuit baking mix, melted butter, beaten eggs, sugar, and salt. Stir until well mixed.

Gently fold the crushed saltine crackers into the mixture, ensuring they are evenly distributed.

Transfer the mixture to the greased casserole dish, spreading it out evenly.

Sprinkle the breadcrumbs evenly over the top of the casserole mixture.

Bake in the preheated oven for about 30 minutes, or until the top is golden brown and the center is set.

Let the casserole cool for a few minutes before serving.