Cook the ravioli according to the package directions. Drain, reserving 3 tablespoons of the cooking water.
Meanwhile, 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, ½ teaspoon salt, ¼ teaspoon pepper, the parsley, and the reserved cooking water.
Add the ravioli and toss to coat. Divide among plates and sprinkle with the Parmesan.