Preheat the oven to 450 degrees.
Place 12 cups of water and 2 tablespoons of salt in a large pot, cover, and bring to a boil. Remove and discard 1½ inches from the woody ends of the asparagus. Peel the bottom 2 inches of each spear with a vegetable peeler (see note). When the water boils, add the asparagus and blanch for 3 to 4 minutes, until al dente. Drain.
Transfer the asparagus to a 10 x 12-inch rectangular baking dish. Add the butter, 1 teaspoon salt and ½ teaspoon pepper and toss to coat the asparagus. Arrange the asparagus decoratively in the dish, sprinkle with the Pecorino and Parmesan, and roast for 5 minutes, until the cheese melts. Squeeze on some lemon juice, sprinkle with fleur de sel, and serve hot.
Note: The peeler works best if you dip it in a glass of water from time to time.
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