Pat the flank steak dry and rub both sides evenly with chili powder, cumin, sea salt, and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Place the steak in the skillet and sear for 3-4 minutes per side for medium-rare, then transfer to a cutting board to rest for 5 minutes.
In the same skillet, add the sliced bell peppers and red onions, sautéing for 3-5 minutes until they are tender and slightly charred.
Steam the cauliflower rice in a separate pan or microwave until tender, then stir in the black beans to warm through.
Thinly slice the rested steak against the grain into bite-sized strips.
Divide the cauliflower rice and bean mixture into a bowl, then top with the sautéed peppers, onions, and sliced steak.
Garnish with fresh avocado slices, chopped cilantro, and a bright squeeze of lime juice before serving.