WHISK eggs and salt together in large bowl until frothy and well combined. Stir in cheese.
PREHEAT broiler. In medium skillet over medium heat, melt butter; add onion and cook, stirring, until translucent.
INCREASE heat to medium-high. Stir egg mixture to redistribute cheese and slowly pour over onion. Reduce heat to very low and cook until egg mixture is nearly set.
PLACE skillet under broiler until top of frittata is set and puffed. Invert onto serving plate and cut into wedges. Serve warm or at room temperature.