This Mother's Day, bake eggs in individual dishes to share with Mom when she wakes up.
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.
Mix up this batter the night before and there will be less work to do in the morning.