1. Preheat the oven to 220°C (425°F).
2. Wash, peel, and slice the zucchini and tomatoes.
Peel and slice the onion.
Slice the goat cheese log.
3. In a gratin dish, arrange 3 rows of zucchini, 2 rows of tomatoes, 1 row of onion, and 1 row of goat cheese, and repeat until the gratin dish is filled.
4. In a bowl, mix olive oil, salt, pepper, and oregano.
Pour the mixture over the gratin.
5. Bake for 25 to 30 minutes.