Bring a saucepan of water to a boil. Prepare a bowl of ice water. Blanch half the parsley leaves in the boiling water for just 20 seconds. Use a slotted spoon to transfer the parsley to the ice water. Drain the parsley, squeezing out the excess water and patting thoroughly dry. In a blender, puree the blanched parsley with 1/3 cup of the olive oil. Transfer to a bowl and season with salt to taste.
Bring a medium saucepan of water to a boil. In a large skillet, toast the buckwheat groats over moderately high heat, shaking the pan often, until lightly browned. Let cool slightly and then add to the boiling water and simmer over low heat. Stir often and cook until just tender but still holding its shape. Allow about five minutes. Drain the buckwheat and spread on a large rimmed baking sheet and let cool to room temperature.
Meanwhile, heat the remaining 2 tablespoons of olive oil in the large skillet. Add your mushrooms and season with salt and pepper to taste. Cover and cook for around five minutes over moderate heat, stirring occasionally, until the mushrooms have released their liquid. Uncover and cook, stirring occasionally, until the mushrooms are golden brown. Allow another five minutes.
In a small bowl, mix the remaining parsley and oil with the balsamic vinegar, lemon juice, garlic and soy sauce.
Using a large bowl, toss the cooked buckwheat, sliced scallions, diced fennel and the parsley dressing. Transfer to a serving dish and then finish with the sautéed mushrooms and a drizzle of your parsley oil. Serve with the lemon wedges.