Easy Philly Cheesesteak Sliders

Name: Easy Philly Cheesesteak Sliders
Ingredients: 1 small yellow onion; 1 small green bell pepper; 1 tablespoon fresh parsley leaves, for garnish (optional); 3 tablespoons unsalted butter; 1/2 teaspoon kosher salt; 1 (12-ounce) package Hawaiian sweet rolls, such as King's; 6 slices American cheese, preferably white; 24 thin slices deli roast beef (about 24 ounces); 6 thin slices provolone cheese