1. Wash the zucchini, then slice them. Cook them in boiling salted water for 6 to 7 minutes, and drain them while still slightly crunchy.
2. Chop the onion and sauté it in a frying pan.
3. Mix the onions and zucchini with drained and crushed tomatoes, olive oil, and minced garlic in a non-stick frying pan; let it simmer for 20 minutes.
4. Preheat the oven to 425°F (210°C).
5. Crumble the feta into coarse crumbs.
6. Grease a gratin dish, arrange the zucchini on top of the tomato mixture.
7. Whisk the eggs, cream, milk, add a little salt and pepper, pour over the vegetables, and sprinkle with feta.
8. Sprinkle with oregano, then bake for 15 minutes at 350°F (180°C) and broil at 400°F (200°C) for 5 minutes.