1. Preheat broiler to high.
2. Cut bread in half lengthwise and again crosswise. Hollow out tops and bottoms of bread, leaving a 1/2-inch-thick shell. Place bread halves on a baking sheet, cut sides up. Broil 1 1/2 minutes or until toasted.
3. Combine milk, flour, and mustard in a medium microwave-safe bowl, stirring with a whisk. Microwave at HIGH 1 1/2 minutes or until thickened, stirring every 30 seconds. Remove from microwave; add 1/8 teaspoon salt and cheese, stirring until cheese melts.
4. Heat a large cast-iron skillet over medium-high heat. Add 1 teaspoon oil to pan; swirl to coat. Add onion, garlic, bell pepper, and 1/8 teaspoon salt; sauté 5 minutes or until tender. Remove onion mixture from pan; keep warm. Add remaining 1 teaspoon oil to pan; swirl to coat. Add beef, remaining 1/8 teaspoon salt, and black pepper to pan; sauté 3 minutes or until tender.
5. Divide beef mixture evenly among bottom halves of bread; top with onion mixture. Drizzle cheese sauce evenly over sandwiches. Top with top halves of bread; cut each sandwich in half.