Slice the flank steak into thin strips against the grain to ensure tenderness.
In a small bowl, whisk together the chili powder, cumin, garlic powder, sea salt, and black pepper.
Rub the spice mixture evenly over the steak strips until well coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the steak strips to the skillet in a single layer and sear for 2-3 minutes until a deep golden-brown crust forms.
Remove the steak from the pan and set aside on a plate.
Add the sliced bell peppers and red onion to the same skillet, sautéing for 4-5 minutes until they are tender and show dark charred spots.
Return the steak to the skillet, drizzle with lime juice, and toss everything together for 1 minute to combine the flavors.
Garnish with fresh cilantro and serve immediately.