In a medium saucepan, sauté onion in oil until golden. Add minced garlic and sauté another few minutes. Pour onion into a nine-inch (20-centimeter) round oven-to-tableware dish, and spread to coat the bottom of the dish.

Using a mandoline, slice all vegetables thinly into rounds (slice the potato last to prevent it from turning brown) and place each one in a separate bowl. Drizzle about one to two teaspoons of oil in each bowl, and sprinkle generously with salt and pepper, to taste. Toss to coat.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

To assemble the tian: Layer all the vegetables, placing them standing vertically around the baking dish, in the following order: yellow squash, potato, zucchini, eggplant, tomato. Keep on layering until an inner and outer ring is formed, taking care to pack very tightly.

Sprinkle the pie with oregano and additional salt and pepper.

To make the sauce: In a small pot, melt margarine. Add flour and stir quickly. Add boiling water and chicken soup mix, whisking to break up any lumps. Cook on medium heat until thickened, hot, and bubbly. Pour sauce over vegetables in the pan and bake for one and a half hours.

Allow to cool and set for about one hour before serving warm or at room temperature.