Cook 1 cup farfalle pasta in boiling, salted water until tender, about 10 minutes. Drain, transfer to a large bowl and toss with 2 1/2 cups cooked and cooled buckwheat; season with salt and pepper. In a large skillet, cook 1 thinly sliced onion in 2 tablespoons extra-virgin olive oil over high heat for 10 minutes. Add 8 ounces sliced mushrooms and season with salt; cook until softened. Serve over the buckwheat mixture; sprinkle with toasted walnuts and chopped parsley.