Prepare the vegetables: Wash the zucchini and slice it thinly. Halve the cherry tomatoes and finely chop the onion. Mince the garlic cloves. Set all the vegetables aside so they are ready to cook.

Cook the zucchini and onion: Heat 2 tablespoons of olive oil in a non-stick oven-safe skillet over medium heat. Add the chopped onion and sauté for 2–3 minutes until it becomes translucent. Add the sliced zucchini and cook for another 3–4 minutes, stirring occasionally, until it softens slightly. Add the minced garlic and cook for 30 seconds until fragrant, taking care not to burn it.

Prepare the egg mixture: In a medium bowl, whisk together the eggs, milk, salt, pepper, oregano, and half of the Parmesan cheese. Make sure the mixture is smooth and slightly frothy for a fluffy frittata.

Combine and cook: Reduce the heat to low and pour the egg mixture evenly over the cooked vegetables in the skillet. Sprinkle the shredded mozzarella and remaining Parmesan on top. Allow it to cook gently on the stovetop for 2–3 minutes, just until the edges begin to set.

Bake the frittata: Preheat your oven broiler to medium-high. Transfer the skillet to the oven and broil for 5–7 minutes, or until the top is golden, slightly puffed, and fully set. Watch carefully to prevent burning.

Slice and serve: Remove the skillet from the oven and let the frittata rest for 2–3 minutes. Use a spatula to carefully loosen the edges, then slice into wedges or rectangular slices. Garnish with fresh basil or parsley before serving. These slices can be enjoyed warm or at room temperature, perfect for a brunch platter, a light lunch, or a snack with some crusty bread on the side.