Slice the sirloin into thin strips and toss in a bowl with the chili powder, cumin, garlic powder, sea salt, and black pepper until evenly coated.
Heat 0.5 tbsp of olive oil in a large cast-iron skillet over medium-high heat until the pan is shimmering.
Add the steak strips to the skillet in a single layer and sear for 2-3 minutes until browned and just cooked through, then remove from the pan and set aside on a plate.
Add the remaining 0.5 tbsp of olive oil to the same skillet, then add the sliced peppers and onions.
Sauté the vegetables for 5-6 minutes, stirring occasionally, until they are tender-crisp and have developed a slight char.
While the vegetables cook, steam the cauliflower rice in a separate small pan or the microwave until tender, then lightly season with a pinch of salt.
Divide the cauliflower rice into a bowl and top with the seared steak and sautéed fajita vegetables.
Finish the bowl by adding the sliced avocado, a drizzle of fresh lime juice, and a sprinkle of chopped cilantro.