Wash tomatoes, remove stalks and cut into thin slices. Peel the onion and cut into thin slices. Arrange both in a shallow bowl. Scatter oregano, salt and pepper over the top. Crumble the cheese and spread on top. Drizzle evenly over the olive oil and let it steep for at least 20 minutes at room temperature.
Sufficient as a side dish for 4 people, otherwise double the amount.