Season the flank steak with chili powder and salt on both sides. Heat 1 teaspoon of oil in a large cast iron skillet over medium high heat. Cook steak 5 to 6 minutes per side to desired doneness. Remove from pan and let stand.
Heat remaining tablespoon oil in skillet and add peppers, garlic, and onion. Cook, stirring often until vegetables soften, about 5 to 6 minutes. Add salsa and cook an additional 2 or 3 minutes.
Slice meat, across the grain, into very thin strips. Heat tortillas according to package directions. Place 1/4 of the meat and 1/4 of the vegetables on each tortilla and serve.