Peel onion and cut into very thin slices. if you have a mandolin this is the time to use it.

Heat 1/2 the olive oil in a small skillet, and over low heat, cook the onion until soft but not browned.

The zucchini should be long, firm to the touch and dark green. With a vegetable peeler, peel off long strips of skin down the length of each squash. The strips should alternate with unpeeled strips. Cut the zucchini into thin even rounds, 1/8 inch thick at most. discard ends.

With a small knife, remove the core from the tomato, then cut them vertically into 1/8 inch slices.

Preheat oven to 475° F. 

Rub the bottom and sides of the shallow casserole with the clove of garlic, pressing down hard to release its juices. 

Over the bed of onion, arrange long overlapping rows of zucchini and tomatoes-forming alternate bands of red and green. season with coarse salt and fresh pepper. 

Scatter thyme leaves over all, and brush the vegetables with the remainder of the olive oil.

Place the dish in a hot pre heated oven and bake for 30-35 minutes. 

After the first 15 minutes check for dryness and baste with rendered juice (tilt casserole to access juice) and if desired drizzle some additional olive oil . 

When finished cooking, the vegetables will be soft and lightly browned.

Sprinkle sea salt and freshly ground pepper