First, preheat the oven. Wash the zucchini, remove the ends, and slice it into rounds. Place the slices on a baking sheet lined with parchment paper, add salt, pepper, and a drizzle of olive oil. Once the oven reaches 300°F (150°C), bake the tray in the oven for 15 minutes.

Peel and slice the onion. After the time has passed, take the tray out of the oven and place the onion slices on top of the zucchini with a pinch of salt. Put the tray back in the oven and increase the temperature to 350°F (180°C). Bake for 10 minutes.

Meanwhile, pour the egg whites into a bowl, add the eggs, Parmesan cheese, milk, and torn basil leaves. Whisk the mixture and season with salt and pepper.

After the time has passed, take the tray out of the oven and place the slices on a plate. Let them cool slightly, then cut them into pieces.

Mix them with the egg mixture and pour into a 10-inch (26 cm) round mold lined with parchment paper.

Bake for 20 minutes at 350°F (180°C). Once the frittata is set, turn off the oven and let it cool slightly. It's delicious to eat either warm or cold.