Roast Beef Sandwich with Caramelized Onions and Grainy Mustard, Horseradish Mayo from Closet Cooking

The best part of the roast beef dinner might just be in using the leftovers to make roast beef sandwiches! Tender roast beef with sweet caramelized onions and the bite of a touch of horseradish.

Servings: makes 4 servings

Prep Time: 10 minutes
Cook Time: 50 minutes
Total Time: 1 hour
  • 1 tablespoon olive oil
  • 4 cups onion, sliced
  • 1/2 cup water
  • 4 buns, cut in half and lightly toasted
  • 4 slices cheese, cheddar, swiss or provolone
  • 1 pound roast beef, warm and thinly sliced
  • 2 tablespoons mayonnaise
  • 1 tablespoon horseradish
  • 1 tablespoon grainy mustard
  • 1/4 cup gravy
  • 1 handful arugula (optional)
  1. Heat the oil in a pan over medium heat.
  2. Add the onions and water and cook, covered, until completely tender and caramelized and golden, about 40-50 minutes, adding more water as needed.
  3. Mis the mayo, horseradish and mustard.
  4. Assemble sandwiches and enjoy!