Slice flat iron steak into thin strips across the grain. In a bowl mix together 3 T. oil, lime juice, garlic, chili powder, cumin, hot pepper flakes, salt and pepper. Add beef strips and stir to coat. Set aside for 30 minutes.
Preheat a large skillet or cast-iron pan to medium-high.
Add 1 T. oil to pan, onions and peppers. Sauté for 3-4 minutes until softened and starting to brown. Transferrer mixture to a bowl and set aside.
Add beef strips to the pan and sauté for 3-4 minutes until meat is browned. Return onions and pepper to skillet. Stir until combined.
To make salsa, place all ingredients in a bowl and stir to combine. Season to taste with salt and pepper.
Serve meat mixture on top of warmed tortillas topped with salsa.