"When you've worked all day you need something that is fast, simple, and versatile…but most importantly delicious. All the butter and juices are absorbed through each layer resulting in a wonderful flavor. Sometimes I leave out the potatoes and add zucchini."
Preheat oven to 375 degrees F (190 degrees C). Spray a 9x13-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.