3 large yellow summer squash, cut into 1/2-inch slices
4 large tomatoes, cut into 1/2-inch slices
1 sweet onion, cut into 1/4-inch slices
2 Tbs butter
Preheat oven to 375 degrees. Spray a 9×13-inch baking pan with cooking spray.
Cover the bottom of the baking pan with a layer of potato slices; season with salt and pepper. Place squash slices on top of the potatoes; repeat seasoning. Lay tomato slices on top of the squash; repeat seasoning. Continue layering the vegetables in this pattern, seasoning each casserole layer. Scatter onion over the casserole; dot with butter.
Bake in the preheated oven until the top is golden brown and the potatoes are soft, about 1 hour.