Pat the sirloin steak dry with paper towels and slice into thin strips against the grain.
In a small bowl, combine the chili powder, smoked paprika, cumin, sea salt, and black pepper.
Toss the steak strips in the spice mixture until every piece is thoroughly and evenly coated.
Heat the olive oil in a large cast-iron skillet over medium-high heat until it begins to shimmer.
Add the steak strips to the skillet in a single layer, searing for 2-3 minutes until a dark golden crust forms.
Remove the steak from the skillet and set aside on a plate to rest.
Add the sliced bell peppers and red onions to the same skillet, sautéing for 4-5 minutes until they are tender-crisp and slightly charred.
Return the steak and any accumulated juices to the skillet, tossing with the vegetables.
Drizzle the fresh lime juice over the mixture and stir to deglaze the pan.
Remove from heat, garnish with chopped cilantro, and serve immediately.