3 tablespoon olive oil
1 white onion, sliced
2 tablespoons balsamic vinegar
1/2 cup water
4 buns cut in half (toasted/broiled)
4 slices provolone
1 lb roast beef, sliced thinly
2 tablespoons mayonaise
1 tablespoon horseradish
1 tablespoon grainy mustard
1 handful arugula
Heat the oil in a pan over medium heat
Add the onions and caramelize them first using the olive oil and then adding the balsamic. (The longer you can break the onions down, the better).
Mix the mayonnaise, horseradish, and mustard.
Butter and broil the buns, pull them out, pile on the beef, add the provolone, put back in the broiler until the cheese is melted, add the onions and horseradish mix and serve.