Mustard Green Gratin-RELOADED

Steps

1. 1Heat oven to 350ºF and place a rack in the middle position.
2. 2Fill a 7 to 8-quart pot halfway with water, add 1 tablespoon of the salt, and bring to a boil over high heat.
3. 3Wash the greens in a very large bowl or sink filled with enough water to cover. Swish around with your hands to dislodge any dirt or sand then leave them to sit for a few minutes so any crunchiness can sink to the bottom.
4. 4Meanwhile, grease a 2 1/2-quart baking dish, casserole, or soufflé dish with 1 tablespoon of the butter and set aside.
5. 5Melt another tablespoon of the butter in a 10-inch skillet over medium-low heat. Add the crushed crackers and cook for 2 minutes, until golden and toasted, Remove to a small bowl and set aside. When the crumbs are cool, stir in 1/4 cup of the ground Parmesan.
6. 6When the water boils, lift the greens out of the wash water and transfer to the pot, pressing down with a spider or tongs to fully submerge. Blanch for 1 minute, then scoop them out, and drain using the inner basket of your salad spinner as a colander. Cool for 5 minutes then spin thoroughly to dry. Transfer to your cutting board, roughly chop, and set aside.
7. 7Place the sherry, mushrooms, and 1 teaspoon of the salt in an 11-inch straight-sided sauté pan over medium-high heat. Cover and cook for 3 minutes to allow the mushrooms to collapse. Remove the lid and continue cooking. When the pan looks almost dry, add 1 tablespoon of the butter and stir with a wooden spoon or spatula to combine. Cook, undisturbed, for 3 minutes, then stir and cook until evenly browned, about 2 minutes.
8. 8Reduce the heat to medium, add the remaining tablespoon of butter and 2 tablespoons of the garlic and cook, stirring constantly until fragrant and golden, about 30 seconds. Add the onions and 1 teaspoon of the salt and cook until translucent and just starting to brown, 2-3 minutes.
9. 9Place the flour in a fine sieve then, tapping against your hand, sift evenly across the vegetables. Cook, stirring constantly, for 1 minute.
10. 10Slowly pour in the milk while constantly stirring and scraping the bottom of the pan. When the mixture begins to thicken and bubble (3-4 minutes), drop the heat to low and stir in the remaining 1/2 cup of the Parmesan, the mustard, hot sauce, nutmeg, black pepper, and the remaining teaspoon of salt. When the cheese has melted, kill the heat and add the remaining garlic and chopped greens. Stir to separate any clumps and to coat with the sauce.
11. 11Transfer the mixture to the buttered dish, and sprinkle the top evenly with the cracker crumb mixture. Bake on the middle rack for 20 minutes.
12. 12Remove the finished gratin to a cooling rack and rest for 10 minutes before serving.