In a small bowl, whisk together the spices for the fajita seasoning.
Heat a skillet over medium heat and the add the ground beef and fajita seasoning. Brown the beef, using a spatula to break it apart into smaller pieces as it cooks. Make sure the seasoning is mixed in with the ground beef.
Once the beef is browned and no longer pink inside, stir in the green chilies (if using). Remove from heat and set aside.
Add oil to a second skillet and heat on high. Add the peppers and onions and sauté for just a few minutes until they start to soften and char slightly.
Serve ground beef, peppers and onions on warmed tortillas with your desired toppings.
Notes
Be careful not to overcook the ground beef as it will turn out tough and chewy. You want it browned and cooked to a safe internal temperature of 160F.
You'll need your skillet hot when you add the peppers and onions. This will help them char quickly without overcooking and getting too soft.
If you only have one skillet, just transfer the cooked ground beef to a plate while you sauté the peppers and onions in the same skillet.
If you're sensitive to spicy food, just leave out the green chilies and use just half of the fajita seasoning. You can always add more to suit your taste once the ground beef is cooked.