This is a great way to use leftover Oven Roast Beef or Grilling Steak -- smart cooks grill an extra steak when barbecuing in order to have one leftover to make a speedy next-day meal like this one. Always let cooked steaks stand for 5 minutes before slicing them across the grain to serve in salads or wraps.
In large bowl, whisk together vinegar, horseradish, canola oil, salt and pepper.
Add cooked beef, lettuce greens, cheese, onion and nuts. Toss together to coat with dressing. Serve.
To toast walnuts, place nuts on large rimmed baking sheet. Toast in 350°F (180°C) oven, until aromatic, stirring once, about 10 minutes.