4 top sirloin steaks
Salt and freshly ground black pepper
1 Tablespoon olive oil
1 Tablespoon unsalted butter
6-8 Portabellini mushrooms or cremini mushrooms, quartered
1 leek, white and light green parts only, sliced lengthwise then thinly sliced crosswise
1 large garlic, minced
1/4 cup balsamic vinegar
2 Tablespoons gf soy sauce
1/2 cup gf
Beef Stock3/4 teaspoon cornstarch