Preheat the oven to 375°F (190°C). Cut the zucchini into half moons.
Heat the olive oil in an oven-safe 9-10 inch (23-25cm) skillet over medium heat.
Add the onion and zucchini.
Cook for 5–6 minutes until softened.
Stir in the garlic and cook for about 30 seconds.
Add the cherry tomatoes.
Cook for another 2 minutes, just until they begin to soften.
In a large bowl, whisk together the eggs, Greek yogurt and Feta.
Add herbs, salt, pepper, and chili flakes, if using.
Pour the egg mixture over the vegetables.
Gently stir once so everything is evenly distributed.
Remove from heat and arrange the tomato slices on top.
Sprinkle salt, freshly ground black pepper and a drizzle of olive oil.
Transfer the skillet to the oven and bake for 20-25 minutes, or until the center is just set and the top is lightly golden.
Let the frittata rest for 5 minutes before slicing. Garnish with extra basil and serve warm or at room temperature.
Leftovers keep well in the refrigerator for up to 3 days and are delicious served cold.