Dice enough onion to yield 1/4 cup. Slice remaining onion to yield 2 cups; separate slices into rings.
Combine water and cornstarch in a small bowl; stir well, and set aside.
Heat 1 teaspoon oil in a medium saucepan over medium-high heat. Add diced onion, and sauté 3 minutes or until tender. Add garlic, and sauté 1 minute. Add cornstarch mixture, 1/4 teaspoon black pepper, and consommé; bring to a boil. Cook sauce 1 minute or until slightly thick, stirring constantly. Remove from heat; keep warm.
Sprinkle both sides of steak with 1/4 teaspoon black pepper and 1/4 teaspoon salt. Place steak on a broiler pan coated with cooking spray; broil 4 to 5 minutes on each side or until desired degree of doneness. Cut steak diagonally across the grain into thin slices.
While steak cooks, heat remaining 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add sliced onion, bell pepper, and mushrooms; sauté 10 minutes or until vegetables are tender and liquid evaporates. Add remaining 1/4 teaspoon black pepper.
Slice bread in half lengthwise, cutting to, but not through, other side. Carefully open bread without separating the halves. Place, cut side up, on a baking sheet. Broil 1 to 2 minutes or until lightly toasted.
Place steak slices and vegetables on bottom half of bread; sprinkle cheese evenly over vegetables. Broil 1 to 2 minutes or until cheese melts. Close sandwich, and cut into 5 equal portions. Serve immediately with warm dipping sauce.