In a heavy cast enamel oval gratin dish, smear the butter all over the bottom and sides.Arrange the slices of zucchini and squash, alternating them. Leave some open space in the middle. Place the cut tomatoes in the middle of the dish.
Preheat oven at 350F.
In a small skillet with 1 teaspoon of olive oil, sauté the onion over medium heat until translucent. Add some fresh thyme while cooking and season with salt and pepper.
When done, 5 to 8 minutes, gently sprinkle them over the zucchini, squash and tomatoes. Season with salt and pepper, fresh thyme and a generous amount of olive oil.
Place in the preheated oven and cook for 15 to 20 minutes.