Mix together the olive oil, cumin, oregano, salt, lime juice and zest, cilantro, garlic, and jalapeño pepper (if using) and whisk well.
Pour half of the marinade over the steak and stir to coat well. Cover and let marinate while you prepare the vegetables.
Remove the stems and cut the red, yellow, and orange peppers in half. Remove the seeds then slice into thin strips.
Cut the onion into thin strips as well.
Set a large skillet over medium-high heat and add about a teaspoon of olive oil to coat the bottom with. Dump in the peppers and onions and cook until soft, about 5 minutes.
Remove the vegetables to a plate and cover with aluminum foil to keep warm.
Next, add the steak strips to the same pan that was used to cook the peppers. Cook until just done, about 5-7 minutes.
Stir the peppers and reserved marinade back into the pan and heat to warm through.
Warm your Mission tortillas in a dry skillet, or microwave oven according to the directions on the back of the package.
Divide the steak fajita mixture between the tortillas, fold in the sides, then roll towards you to close.