Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 1 quart casserole dish.
Alternate slices of the squash and tomatoes in the prepared casserole dish, and sprinkle with green onions. Mix the vinegar, oil, mustard, salt, and pepper in a bowl, and drizzle over the vegetables.
Bake 15 minutes in the preheated oven, or until squash is tender. Cool 15 minutes before serving.