In a small bowl, whisk together the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper to create a marinade.
Place the flank steak in a shallow dish and coat thoroughly with the marinade; let it sit for at least 15 minutes.
While the steak marinates, heat a non-stick skillet over medium heat and lightly sauté the cauliflower rice with the cooked brown rice until heated through and well combined.
Remove the rice blend from the pan and set aside. In the same skillet, add the sliced bell peppers and red onion, sautéing until they are tender-crisp and slightly charred.
Increase the heat to medium-high and add the olive oil. Sear the flank steak for 3-4 minutes per side for medium-rare, then let it rest for 5 minutes before slicing against the grain.
Assemble the bowl by layering the rice blend, sautéed vegetables, and sliced steak. Top with fresh avocado slices and a sprinkle of cilantro.