Chopped Steak Sandwiches

• 2 tablespoons ketchup
• 1 tablespoon Dijon mustard
• 1 jarred hot cherry pepper, stemmed seeded and chopped
• 1 teaspoon red wine vinegar
• 1 lb beef sirloin, tips cut into 1-inch pieces (flap meat)
• kosher salt & freshly ground black pepper
• 3 tablespoons olive oil
• 1 small yellow onion, halved and thinly sliced
• 2 cups sliced white mushrooms (6 ounces)
• 1 1/2 cups shredded sharp cheddar cheese
• 4 long rolls, split and lightly toasted
• 1 1/2 ounces baby arugula (optional)

1. Mix the ketchup, mustard, cherry pepper and vinegar in a small bowl and set aside.
2. In a food processor, pulse half of the beef until just coarsely chopped. Don't skip this quick-chop step. Transfer to a bowl. Repeat with the remaining beef. Toss the beef with 1/2 teaspoons salt and 1/2 teaspoons pepper.
3. Heat 1-1/2 Tbs. of the oil in a heavy 12-inch skillet over medium-high heat until shimmering hot. Add the beef and cook, stirring occasionally, until it loses most of its raw color, about 3 minutes. Careful not to overcook. Transfer to a plate. Heat the remaining 1-1/2 Tbs. oil in the pan. Add the onion and 1/4 teaspoons salt; cook, stirring often, until beginning to soften and brown, about 4 minutes. Add the mushrooms and 1/4 teaspoons salt; cook, stirring often, until they soften and begin to release their juice, about 2 minutes. Add the ketchup mixture and beef and any accumulated juices. Cook, stirring often, until heated through, about 2 minutes. Stir in the cheese until melted, about 1 minute more.
4. Split the rolls almost through lengthwise and pile on the steak and arugula. Cut the sandwiches in half and serve.

ketchup, mustard, pepper, red wine vinegar, beef sirloin, kosher salt, olive oil, yellow onion, white mushrooms, cheddar cheese, long rolls, baby arugula

Taken from www.food.com/recipe/chopped-steak-sandwiches-520929 (may not work)