Stir together all dressing ingredients in medium bowl; set aside.
Spray 12-inch skillet with no stick cooking spray; add beef and onions. Cook over medium heat, stirring occasionally, until onions just begin to soften and beef is heated through (2 to 3 minutes).
Toss together beef mixture, spinach, tomatoes and dressing in large bowl. If desired, serve with croutons or garlic toast.