Cheese, Leek, and Herb Souffle Casserole

Heat 1 tablespoon butter in a large skillet over medium-high heat until pale gold, about 2 minutes. Add leeks and cook, stirring frequently, until softened and bright green, about 3 minutes. Season with salt and pepper; let cool. (Leeks can be made 1 day ahead and refrigerated.)

Butter a 9-by-13-inch baking dish. Cut strips of parchment 4 inches wide and long enough to form a collar around inside of dish. Butter 1 side of parchment, and press unbuttered side against inside of dish. (Ends should overlap, and parchment should extend 2 inches above top of dish.) Place 1/2 cup Parmesan in dish, and coat sides well. Pour out and discard excess cheese.

Melt 1 stick butter in a heavy saucepan over medium heat. Add flour, and cook, whisking constantly, until well combined and thoroughly heated. Remove pan from heat, and gradually add milk, whisking constantly until batter is very smooth before adding more milk. Add 1 teaspoon salt, and season with pepper. Return pan to medium heat, and whisk until souffle base is smooth, shiny, and thickened, about 12 minutes.

Preheat oven to 400 degrees. Transfer souffle base to a bowl, and beat in egg yolks, 1 at a time, with a mixer on medium speed, increasing speed to high for 10 seconds after each addition to thoroughly incorporate. After last yolk is added, beat 20 seconds on high speed, then scrape down side of bowl. Add Gruyere, the remaining Parmesan, the leeks, and herbs, and beat until well combined.

Beat egg whites with 1/2 teaspoon salt until stiff peaks form. Fold whites into souffle base, one third at a time, folding gently but thoroughly (a few streaks of whites are fine). Spoon souffle mixture into dish, and bake until puffed and golden, 35 to 40 minutes.