Preheat a fan oven to 180°C (356°F).
Cut the chicken breast, zucchini, red pepper, and white onion into small, even cubes.
Heat the sunflower oil in a non-stick frying pan over medium heat and fry the chicken and vegetables until the chicken is cooked through and vegetables are softened; set aside.
In a mixing bowl, whisk the eggs until combined, then stir in the yogurt and grated cheese.
Gradually whisk in the flour until smooth, then season the egg mixture with salt and pepper to taste.
Place the fried chicken and vegetables in a heat-resistant casserole (about 20×30 cm) and spread them evenly.
Pour the egg, yogurt, cheese, and flour mixture over the chicken and vegetables, spreading it out evenly.
Bake in the preheated oven for about 25 minutes, until the frittata is set and lightly golden on top.
Remove from the oven and serve hot or cold.