Toasting the walnuts deepens their flavor. The olive oil they're cooked in is used to dress the salad.
Combine walnuts and oil in a small saucepan over medium-low heat. Cook, stirring occasionally, until walnuts aretoasted, about 15 minutes.
Toss spinach with vinegar, and season with salt and pepper.Top with walnuts, and drizzle with warm olive oil.