Cook the Potatoes. Heat olive oil or butter in a large oven-safe skillet over medium heat. Add sliced potatoes and cook for 8-10 minutes, stirring occasionally, until they are golden brown and tender. Stir in the onions and garlic, cooking for another 2-3 minutes until fragrant.
Whisk the Eggs. In a large bowl, whisk together eggs, milk, salt, black pepper, and smoked paprika until fully combined. If using cheese, stir it into the egg mixture.
Combine and Cook. Reduce heat to low and pour the egg mixture over the potatoes in the skillet. Gently shake the pan to evenly distribute the eggs around the potatoes. Cook uncovered for 4-5 minutes, until the edges begin to set but the center is still slightly runny.
Finish in the Oven. Transfer the skillet to a preheated 375°F oven and bake for 10-12 minutes until the frittata is fully set. If you prefer a golden top, switch the oven to broil for 1-2 minutes at the end.
Serve and Enjoy. Remove from the oven and let the frittata cool for a few minutes. Garnish with fresh herbs and slice into wedges. Serve warm or at room temperature.