1. Preheat the oven to 375°F (190°C).

2. Prepare all vegetables: thinly slice zucchini, chop onion, halve tomatoes, and mince garlic.

3. Heat olive oil in an oven-safe skillet over medium heat. Sauté onion until soft, about 3 minutes.

4. Add garlic and zucchini, cook until zucchini is tender and moisture has evaporated, about 5 minutes.

5. Add cherry tomatoes and cook for another 1–2 minutes. Remove from heat.

6. In a large bowl, whisk together eggs, milk or cream, salt, pepper, parsley, and crumbled feta.

7. Pour the egg mixture evenly over the vegetables in the skillet.

8. Return skillet to medium-low heat and cook without stirring for 4–5 minutes, until the edges are set.

9. Transfer skillet to the oven and bake for 10–12 minutes, or until the center is set and the top is lightly golden.

10. Let the frittata cool for 5 minutes before slicing and serving.