Place a nonstick skillet over medium-high heat until hot. While skillet heats, combine chili powder and garlic powder; sprinkle over both sides of steak. Add steak to skillet; cook 4 minutes on each side. Remove steak from skillet; let stand 5 minutes. Slice steak diagonally across grain into 1/4-inch slices.
While steak stands, arrange pepper strips and onion on bread shell. Arrange steak strips over vegetables. Spoon salsa over steak. Bake at 450° for 8 minutes; sprinkle with cheese. Bake 5 additional minutes.