Greens-Filled Ravioli with Walnuts

Cook Swiss chard in a large pot of boiling salted water 5 minutes, then transfer to a colander, reserving boiling water in pot. Rinse chard under cold water. Blanch escarole in same boiling water 3 minutes, then drain in colander and rinse with cold water. Squeeze excess water from greens with your hands and cut into 3/4-inch pieces. Cook onion in oil in a 10-inch heavy skillet over moderately high heat, stirring occasionally, until pale golden. Add garlic and cook, stirring, until fragrant, about 30 seconds. Add greens and currants and cook, stirring, until any liquid is evaporated. Stir in zest, juice, parmesan, and salt to taste. Cool to room temperature. Working with 1 wrapper at a time and keeping remaining wrappers in plastic wrap, mound 1 teaspoon filling in center. Moisten edges of wrapper with water and fold wrapper in half over filling, pressing down around filling to force out air. Seal edges well, pressing them together with your fingertips. Arrange ravioli as filled in 1 layer on a wax paper–lined baking sheet and keep covered with plastic wrap. Heat butter in a 12-inch nonstick skillet over moderate heat until foam subsides, then cook walnuts, stirring occasionally, until fragrant and golden, about 3 minutes. Remove from heat and season with salt. Cook ravioli in a 5-quart pot of boiling salted water 2 minutes and transfer with a slotted spoon to skillet with walnuts. Reserve 1 cup pasta cooking liquid. Add parsley and salt and pepper to taste to ravioli and toss to coat over moderately high heat, adding enough cooking liquid, 1/4 cup at a time, to keep ravioli from sticking together.