Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish. While the oven preheats, cut the yellow squash into 1/2-inch thick half-moons, slice the zucchini into 1/2-inch thick rounds, and dice the onion into roughly 1/4-inch pieces. Crush the Ritz crackers into bite-sized crumbs (not too fine—you want texture). Grate the 1 1/2 cups of cheddar cheese if not already done, and measure out all remaining ingredients so everything is ready to go.

In a large pot, bring the chicken broth to a boil over medium-high heat. Add the prepared squash, zucchini, and onion, then reduce heat to medium and simmer for 6-7 minutes until the vegetables are just tender but not mushy. Drain the vegetables thoroughly in a colander, discarding the broth. Stir the drained vegetables together with the corn in a large bowl.

In a medium bowl, whisk together the mayonnaise, sour cream, and room-temperature eggs until smooth and fully combined. This creates a silky base that will bind everything together. Stir in the melted butter, salt, black pepper, garlic powder, and fresh thyme leaves until evenly distributed. Finally, fold in the 1 1/2 cups of freshly grated cheddar cheese. I like to reserve a bit of this creamy mixture to swirl through the middle of the casserole for extra richness.

Pour the creamy binder from Step 3 over the vegetable and corn mixture from Step 2, stirring gently until everything is evenly coated. Spread half of this combined mixture into the prepared baking dish, creating an even layer. Sprinkle half of the cracker crumbs over this first layer, then add the remaining vegetable mixture on top. Finally, top with the remaining cracker crumbs and the 1/2 cup of additional cheddar cheese for a golden, cheesy finish.

Bake in the preheated 350°F oven for 25 minutes until the top is golden brown and the casserole is bubbling around the edges. The cheese should be melted and the crackers lightly crisped. Let the casserole rest for 5 minutes before serving—this helps it set slightly and makes portioning cleaner.