For the filling
01 -
1 tablespoon olive oil
02 -
1 lb (450 g) 80/20 ground beef
03 -
1 small yellow onion, finely diced
04 -
1 green bell pepper, finely diced
05 -
1 cup (75 g) mushrooms, finely chopped
06 -
2 cloves garlic, minced
07 -
1/2 teaspoon kosher salt, plus more to taste
08 -
1/2 teaspoon black pepper
For the sauce and sandwiches
09 -
2 tablespoons ketchup
10 -
1 tablespoon Worcestershire sauce
11 -
1/2 cup (120 ml) low-sodium beef broth
12 -
1 tablespoon cornstarch
13 -
6 slices provolone cheese
14 -
4 hamburger buns or potato rolls
15 -
1 tablespoon softened butter, for toasting buns