Preheat oven to 350 degrees F.
Thinly slice the yellow squash and the zucchini
Heat a skillet to medium/high heat. Add 1 tbs of the olive oil and saute the sweet onion.
Spray an 8x8-inch baking dish with vegetable spray.
Alternate the yellow squash and the zucchini in a single layer. Add 1/3 of the sauteed sweet onions. Then sprinkle with a handful of the cheese.
Continue with two more layers and a final layer of the zucchini and yellow squash.
Beat the eggs in a bowl and then drizzle beaten eggs over the squash.
Sprinkle the top with bread crumbs. Melt the remaining butter and pour it over the top.
Bake the casserole for 60-75 minutes, or until the top is nicely browned and the zucchini is soft and tender.
Serve immediately.