Preheat oven to 350 degrees. Meanwhile, in a small bowl or jar, whisk or shake vinegar and oil until thick and combined; season with salt and pepper. (Dressing can be made a day ahead and refrigerated, covered.)
Place nuts on a rimmed baking sheet. Bake until golden brown, 7 to 10 minutes.
In a large bowl, combine watercress, frisee, pears, and dressing; toss gently to coat. Divide among plates; sprinkle with walnut pieces.