Preheat the oven to 375°F (190°C).
In a large mixing bowl, crack the eggs and whisk them until fully blended. Season with salt and black pepper to taste.
In a non-stick skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 2-3 minutes until it becomes translucent.
Add the sliced zucchini to the skillet and continue to cook for another 3-4 minutes until tender.
Stir in the diced tomato and cook for an additional 2 minutes to allow the flavors to meld.
Pour the egg mixture over the vegetables in the skillet, making sure the ingredients are evenly distributed.
Sprinkle the shredded cheese on top.
Cook on the stovetop for about 5 minutes until the edges start to set, then transfer the skillet to the preheated oven.
Bake for 10-12 minutes, or until the center is set and the top is slightly golden.
Remove from the oven and let it cool for a few minutes. Garnish with fresh basil leaves, if desired, and cut into wedges to serve.