Preheat your oven to 325°F (165°C) and grease a 1½-quart casserole dish.

In a large pot, boil the sliced yellow squash, zucchini, and diced onion for about 15 minutes until tender. Drain thoroughly.

In a mixing bowl, combine drained vegetables with cheddar cheese, biscuit mix, melted butter, beaten eggs, sugar, and salt. Stir until evenly mixed.

Gently fold in crushed saltine crackers to the mixture.

Transfer the mixture into the prepared casserole dish and spread evenly.

Sprinkle breadcrumbs over the mixture and add extra cheese if desired.

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

Let cool for a few minutes before serving to allow flavors to settle.