Flank Steak Sandwiches

Ingredients

• 1 cup chopped onion
• 1 cup dry red wine or beef broth
• 3/4 cup reduced-sodium soy sauce
• 1/2 cup olive oil, divided
• 4-1/2 teaspoons minced garlic, divided
• 1-1/2 teaspoons ground mustard
• 1-1/2 teaspoons ground ginger
• 1 beef flank steak (1-1/2 pounds)
• 1 medium sweet red pepper, cut into 1-inch strips
• 1 medium sweet yellow pepper, cut into 1-inch strips
• 1 medium red onion, thickly sliced
• 1/4 teaspoon pepper
• 6 French rolls, split