Cook the ravioli according to the package directions. Drain, reserving 3 tablespoons of the cooking water.
Heat the oil in a medium skillet over medium heat. Add the garlic and walnuts. Cook, stirring, until the nuts are lightly toasted and fragrant, about 5 minutes. Stir in the lemon juice, 1/2 teaspoon salt, 1/4 teaspoon pepper, the parsley, and the reserved cooking water. Add the ravioli and toss to coat. Divide among individual plates and sprinkle with the Parmesan.