The classic vinaigrette is very simple -- oil, vinegar, salt, pepper and herbs. Use as a dressing on salads or as a marinade.
Mix all ingredients in medium bowl with wire whisk until well blended. Cover.
Refrigerate until ready to serve.
Mix 1 package (6 ounces) baby spinach leaves, 1 cup sliced mushrooms and 1/2 cup thinly sliced red onion in large bowl. Drizzle with Italian Vinaigrette; toss lightly to coat.