Slice the chicken breast into thin strips and toss in a bowl with the ground cumin, chili powder, sea salt, and black pepper until evenly coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Add the chicken strips to the hot skillet and sear for 5-6 minutes, turning occasionally, until golden brown and cooked through.
Remove the chicken from the skillet and set aside on a plate.
In the same skillet, add the sliced bell peppers and red onions, cooking for 4-5 minutes without stirring too often to allow a dark, smoky char to develop.
Warm the whole wheat tortilla in a separate dry pan or directly over a low gas flame for 15 seconds per side.
Assemble the fajita by placing the charred vegetables and chicken onto the warm tortilla.
Top with sliced avocado, fresh cilantro, and a generous squeeze of lime juice before serving.