5 tablespoons butter, plus more for pan1 tablespoon grated Parmesan 2 fat bulbs green garlic, root and green parts trimmed, outer layer removed 1/4 cup flour 2 cups milk 1/4 teaspoon dried thyme 3/4 teaspoon kosher salt, plus a pinch 1/4 teaspoon ground black pepper 1/8 teaspoon grated nutmeg 4 egg yolks and 6 egg whites 2/3 cup grated Gruyère cheese 1/4 cup chopped chives
Preheat oven to 400 degrees. Butter a 2-quart gratin dish and sprinkle bottom and sides with Parmesan. Using a sharp knife or food processor, mince garlic.
Melt butter in a saucepan and let cook for 1 minute. Add flour and cook, whisking, until mixture is pale golden, about 5 minutes. Whisk in the milk and dried thyme, and continue to cook, whisking constantly, until mixture is thick and smooth, about 2 minutes longer.
Turn off heat and whisk in salt, pepper and nutmeg. Transfer to a large bowl and whisk in egg yolks, one at a time. Whisk in minced garlic, cheese and chives.
In a mixer, whip egg whites with a pinch of salt until they hold soft peaks. Using a spatula, fold a third of the whites into yolk mixture to lighten it, then fold in remaining whites, taking care not to overmix..
Spread mixture in prepared pan and smooth top. Bake until golden brown and puffed, 20 to 25 minutes.