1 (12-ounce) flank steak, trimmed
1/4 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
2 (5-inch) portobello mushroom caps
2 teaspoons extra-virgin olive oil, divided
1 cup thinly sliced onion
1 1/2 cups thinly sliced green bell pepper
2 teaspoons minced garlic
1/2 teaspoon Worcestershire sauce
1/2 teaspoon lower-sodium soy sauce
2 teaspoons all-purpose flour
1/2 cup 1% low-fat milk
1 ounce provolone cheese, torn into small pieces
2 tablespoons grated Parmigiano-Reggiano cheese
1/4 teaspoon dry mustard
4 (3-ounce)