Place the steak slices in a large mixing bowl, a baking dish, or a freezer bag.
Whisk together the lime juice, olive oil, garlic, cumin, chili powder, and salt.
Pour the marinade over the steak slices, mix around until evenly coated, and cover. Refrigerate for 15 minutes or up to 24 hours.
Remove steak from fridge and set aside to come to room temperature.
Cut up four 12 x 12 aluminum foil sheets and set aside.
Place the sliced bell peppers and sliced onions in a large mixing bowl and season with olive oil, oregano, garlic powder, paprika, salt, and pepper; mix to combine.
Portion out the peppers mixture and arrange in the center of each foil sheet.
Remove the steak from the marinade; portion out the steak slices and arrange over the vegetables inside the foil packs. Discard the marinade.
Close the foils and wrap them up tightly around the steak and vegetables.
Arrange the foil packs on a baking sheet and cook for 15 minutes or until done. I suggest cooking for 13 minutes in the oven, then opening up the packets and putting them under the broiler for a couple of minutes to get that bit of browning on top.
You can also grill the steak fajitas foil packs over high heat for around 7 minutes per side. An instant-read thermometer should read 125˚F in the center for medium rare and 135˚F for medium.
Remove from oven or grill and serve with flour tortillas, avocado slices, lime wedges, sour cream, etc.
Notes
Storage: Transfer leftovers to airtight containers and refrigerate for 3 days. You can freeze the steak and vegetables in separate airtight containers or freezer bags for 2 to 3 months.