Stir in the parmesan, ¾ cup manchego, and ½ cup of breadcrumbs. Add the eggs, salt, and pepper and stir again. Transfer mixture to prepared baking dish.
In a small bowl, mix together the remaining ¼ cup manchego and ½ cup bread crumbs. Melt the remaining 1 tablespoon of butter and drizzle over the breadcrumbs, tossing to ensure breadcrumbs are coated evenly. Sprinkle the breadcrumbs mixture over the summer squash mixture.
Bake for 30 minutes, until the top becomes crisp and golden brown.