Philly Cheesesteak Sloppy Joes with Peppers and Melted Cheese

Ingredients

• 4 hamburger buns or brioche buns
• 2 tablespoons (28 g) unsalted butter, softened
• 1 tablespoon olive oil
• 1 lb (450 g) ground beef, 85/15 preferred
• 1 small yellow onion, diced
• 1 green bell pepper, diced
• 1 red bell pepper, diced
• 1 cup (75 g) sliced mushrooms, chopped
• 2 cloves garlic, minced
• 2 tablespoons ketchup
• 1 tablespoon Worcestershire sauce
• 1/2 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 1/2 cup (120 ml) low-sodium beef broth
• 1 tablespoon cornstarch
• 6 slices provolone cheese
• 1 tablespoon chopped parsley, optional