Melt butter in a large skillet over medium high heat. Skim off 2 Tablespoons and set aside.
Add onion and cook until softened but not brown, about 5 minutes.
Add squash, stir well, cover and cook about 7-8 minutes until soft but not brown.
Transfer to bowl and let cool for a few minutes.
Stir in Parmesan cheese, 1/4 cup of the manchego, 1/2 cup of the breadcrumbs.
Stir in the eggs, salt and pepper and transfer mixture to a buttered 8X8 baking dish.
Cover and refrigerate at this point if desired.
In a small bowl, combine the remaining manchego, bread crumbs, and reserved melted butter. Toss with a fork to combine and sprinkle over the top of casserole.
Bake at 350 degrees for 30 minutes {longer if dish is cold} until top is crisp and golden and a knife inserted in the center comes out clean. Serves 4-6