Mix together the olive oil, lime juice, garlic, brown sugar, chili powder, cumin, red pepper flakes, salt, smoked paprika, oregano, and cilantro. Whisk well.
Place the steak strips into a large bowl and pour the marinade over the top. Stir gently to coat, then set aside.
Set a large skillet over medium-high heat and add the olive oil.
Add the strips of peppers and onion and cook until soft and just starting to blister, about 5 minutes. Remove to a plate.
Next, add the steak strips to the same pan that was used to cook the peppers and raise the heat to high (unless you’re using flat iron, then leave it at medium-high).
Cook until just done through, about 5 minutes.
Stir the peppers back into the pan and heat to warm through.
Divide the steak fajita mixture between the tortillas and enjoy.