Pat the flank steak dry with a paper towel and slice it into thin strips against the grain.
In a small bowl, combine the chili powder, cumin, garlic powder, sea salt, and black pepper.
Toss the steak strips with half of the spice mixture until evenly coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the steak to the skillet in a single layer and sear for 2 minutes per side until charred and cooked through, then remove and set aside.
Add the sliced bell peppers and red onion to the same skillet, seasoning with the remaining spice mixture.
Sauté the vegetables for 5-6 minutes until tender-crisp with visible charred edges.
Return the steak to the skillet, drizzle with lime juice, and toss everything together for 1 minute.
Garnish with fresh chopped cilantro before serving hot.