Caramelized Onion and Steak Sandwiches with White Cheddar Sauce

Name: Caramelized Onion and Steak Sandwiches with White Cheddar Sauce
Cuisine: American
Ingredients: 1 to 1½ cups evaporated milk; 8 ounces sharp white cheddar cheese, grated; 1 tablespoon cornstarch; ½ teaspoon ground mustard powder; Olive oil; 1 pound chuck steak, sliced very thin; Salt and pepper, to taste; 2 large onions, caramelized (click here for instructions) and finely chopped; 1 teaspoon sugar; 2 teaspoons Worcestershire sauce; Butter, for serving; Rolls, for serving