How to Make It
Preheat oven to 325°F. Place walnuts, hazelnuts, and almonds on a rimmed baking sheet. Bake in preheated oven until nuts are toasted, about 6 minutes.
Place toasted nuts and olive oil in a food processor; pulse until nuts are finely chopped and a paste begins to form, about 10 pulses. Set nut butter aside.
Remove and discard stems from kale. Tear kale into bite-size pieces (about 20 cups). Heat a large Dutch oven over high. Add kale, and cook, stirring constantly, 10 seconds. Add 1/2 cup water and butter, and cook, stirring occasionally, until kale is just tender and still bright green, about 3 minutes. Remove from heat, and toss with nut butter. Stir in lemon juice and salt. Transfer to a platter, and sprinkle with ricotta salata.
Cavolo nero is also sold as Tuscan, lacinato, and dinosaur kale.