Ingredients
1 cup all-purpose flour (spooned and leveled)
Coarse salt and ground pepper
2 large eggs, separated
1 1/4 cups whole milk
2 tablespoons unsalted butter, plus more for ramekins
1/2 cup finely diced yellow onion
3 ounces ham, diced medium (1/2 cup)
2 ounces Gruyere, grated (1/2 cup)
2 tablespoons chopped fresh chives
Unsalted butter
Preparation
In a medium bowl, sift together flour, 1 teaspoon salt, and 1/4 teaspoon pepper. Make a well in center of flour mixture; whisk egg yolks and 1/4 cup milk into flour mixture, then gradually add 1 cup milk to form a smooth batter. Let rest at least 30 minutes (or refrigerate in an airtight container overnight).
Preheat oven to 425 degrees. Butter six 4-ounce ramekins. In a medium skillet, melt butter over medium. Add onion and cook until translucent, 8 minutes; let cool 10 minutes. Divide onion, ham, and Gruyere among ramekins.
Whisk egg whites until soft peaks form. Gently fold into batter with a rubber spatula. Spoon over onion mixture and sprinkle with chives. Place ramekins on a rimmed baking sheet and bake until golden and puffed, 18 to 20 minutes. Serve warm.
