Melt butter and oil in a 10” non-stick skillet over medium heat.
Add onions, sprinkle with salt and pepper, and sauté 5 minutes or until soft.
Add chopped fries and sauté 3 minutes. Break up any larger pieces with a rubber spatula.
Distribute fries and onions evenly in the pan and reduce the heat to medium-low.
Slowly pour in the eggs. Shake the pan gently to distribute, tilt the pan slightly, and use the spatula to lift the edges to allow raw egg to roll towards the edges to cook.
When frittata is nearly fully cooked, cover with a lid and let steam for 1 minute to finish cooking the eggs.
Gently slide onto a large plate and serve immediately.