Preheat your oven to 375°F (190°C).
In a large bowl, whisk together the eggs, milk, salt, and pepper until well combined.
In a large oven-safe skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until softened.
Add the grated potato to the skillet and cook for another 5 minutes, stirring occasionally, until the potato starts to soften.
Stir in the grated zucchini and cook for an additional 3-4 minutes until the zucchini is tender.
Pour the egg mixture over the vegetables in the skillet, ensuring even distribution. Sprinkle the shredded cheddar cheese on top.
Cook on the stovetop for about 2-3 minutes until the edges begin to set, then transfer the skillet to the preheated oven.
Bake for 15-20 minutes, or until the frittata is puffed and golden brown in the center.
Remove from the oven and let it cool for a few minutes before slicing. Garnish with fresh parsley before serving.