Slice steak into thin strips across the grain. In a large bowl, mix together 3 tablespoons of oil, lime juice, garlic, chili powder, cumin, red pepper flakes, salt, and pepper. Add steak strips and stir to coat, then set aside for 30 minutes.
Preheat a large skillet or cast-iron pan to medium-high.
Add 1 tablespoon of oil to skillet/pan, then add onions and peppers. Sauté for 3-4 minutes, until softened and starting to brown. Transfer mixture to clean large bowl and set aside.
Add steak strips to pan and sauté for 3-4 minutes until browned. Return onions and pepper to skillet and stir until well combined.
Salsa
Place all ingredients in a medium bowl and stir to combine. Season to taste with salt and pepper.
Plating
Serve steak mixture over warmed tortillas and top with salsa.