Cheese, Leek, and Herb Souffle Casserole

1 T. plus 1 stick unsalted butter, plus more for dish

1 pound large leeks (white and pale-green parts only), quartered lengthwise, cut into 1/8 inch slices (4 cups), and rinsed well

Coarse salt and freshly ground pepper

2 ounces Parmesan cheese, finely grated (1/2 cup plus 1/2 cup)

½ cup all-purpose flour

2 ½ cups whole milk

6 large eggs, separated

5½ ounces Gruyere cheese, grated (1½ cups)

1 T. finely chopped parsley

1 T. finely chopped fresh dill

1 T. finely chopped fresh chives

1. Heat 1 T. butter in a large skillet over medium-high heat until pale gold, about 2 minutes.
2. 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.
3. Butter a 9x13" baking dish.  Cut strips of parchment 4 inches wide and long enough to form a collar around inside of the dish.  Butter one side of parchment, and press unbuttered side against inside of the dish.  Ends should overlap and parchment should extend 2 inches above top of the dish.
4. Place 1/2 c. Parmesan in dish and coat all sides well.  Pour out and discard excess cheese.
5. Melt 1 stick butter in a heavy saucepan over medium heat.
6. Add flour and cook, whisking constantly, until well combined and thoroughly heated.
7. Remove pan from heat and gradually add milk, whisking constantly until the batter is very smooth before adding more milk.
8. Add 1 t. salt and season with pepper.
9. Return pan to medium heat and whisk until souffle base is smooth, shiny and thickened about 12 minutes. Let cool slightly.
10. Preheat oven to 400°.
11. 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.
12. After last egg yolk is added, beat 20 seconds on high speed.  then scrape down side of bowl.
13. Add Gruyere, the remaining Parmesan, the leeks and herbs and beat until well combined.
14. Beat egg whites with ½ t. salt until stiff peaks form.
15. Fold whites into souffle base, 1/3rd at a time, folding gently but thoroughly (a few white streaks are OK).
16. Spoon souffle mixture into dish and bake until puffed and golden brown, about 35 to 40 minutes.