How to Make It
Separate the egg whites and yolks and place in two medium-sized bowls. Whisk egg whites until soft peaks form (when you hold up the whisk, the peaks should droop over but not fall back into the bowl). By hand, this will take about 5 minutes, but you can also use a hand or stand mixer. Add a pinch of kosher salt and a few grinds black pepper to the egg yolks. Add half the egg white mixture to the yolks and fold until just combined. Pour the mixture back into the bowl with remaining egg whites and fold until just combined. Gently fold in the Gruyère and chives.
Preheat the broiler with rack in the highest position. Melt butter in a 9-inch oven-proof skillet over high heat until melted and sizzling, but not browned. Pour in the egg mixture and cook until the edges begin to set and the bottom is lightly browned, about 3 minutes. Sprinkle with Parmesan, then place under broiler until puffy and browned on top, about 1 minute. Sprinkle with chives and serve immediately.