In a small bowl, whisk together 3/4 cup soy sauce, 1/4 cup packed brown sugar, 4 tablespoons balsamic vinegar, 1/2 teaspoon dry mustard powder, 1/2 teaspoon red pepper flakes, 1/4 teaspoon black pepper and a Pinch of salt.
Place 1 2 lb. flank steak in a shallow dish or large zip-top bag and add soy sauce mixture. Cover the dish or seal the bag and refrigerate for at least 4-6 hours, or overnight.
Heat grill to high heat. Grill steak, covered, for 6 minutes per side, or until medium rare. Let stand 10 minutes before slicing into thin strips to serve.
To make Herbed Gorgonzola Butter, combine 1 cup unsalted butter, 1 cup crumbled Gorgonzola cheese, 3 cloves garlic, 1/3 to 1/2 cup loosely packed fresh herbs, and Salt and pepper, to taste, in a food processor and pulse until smooth. Spread over warm flank steak.