Blanch kale in boiling salted water for about 3 minutes; drain and refresh in cold water. Squeeze out excess water and chop leaves coarsely

Heat olive oil in a large pot and add onion strips. Sauté for about 4 minutes, stirring often. Add salt and pepper and continue cooking, stirring often, until onions begin to color. Add currants and stir for about 1 minute. Add kale and stir well. Cook, stirring frequently, until kale is tender. Remove from heat. Stir in lemon juice and sprinkle with hazelnuts. Serve at once.