In a medium bowl whisk all fajita seasoning/marinade ingredients. (Reserve about 2 tablespoons of marinade for the veggies.) Add steak strips and toss to coat with the marinade. In a separate, small bowl toss the cut veggies with the marinade. Cover and refrigerate for one hour. Alternatively, sealable plastic bags can be used here as well.
In a nonstick skillet/pan or cast-iron skillet over medium-high heat, add olive oil, and once heated cook the veggies until softened but still crisp. About 5 minutes.
Set aside. Wipe the skillet if needed, add more oil and cook the steak strips for 5-6 minutes. They will cook very fast as they are small and cut into strips.
Return the veggies to the pan and combine everything. Taste for seasoning, drizzle with lime juice to taste right before serving.
Warm the taco shells or tortillas one by one in a separate dry pan until they begin to char (this will happen very fast, about 30 seconds per side on medium-high heat). Wrap them in foil to keep warm until ready to use.
Serve the steak fajita in taco shells with sour cream, lime wedges, fresh sliced green onions, fresh cilantro, more cayenne and freshly cracked black pepper, avocados, shredded cheese.