Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Trim the ends of the Brussels sprouts and cut them in half vertically, then place them on the prepared baking sheet.
Drizzle the sprouts with 0.25 tbsp of olive oil and season with a pinch of sea salt and black pepper, tossing to coat evenly.
Roast the Brussels sprouts for 20-25 minutes, tossing halfway through, until they are tender and the outer leaves are golden and crispy.
While the sprouts roast, pat the steak dry with paper towels and season both sides with the remaining salt, pepper, and garlic powder.
Heat a cast-iron skillet over medium-high heat with the remaining 0.25 tbsp of olive oil until the pan is shimmering.
Place the steak in the hot skillet and sear for 3-5 minutes per side, depending on thickness, until it reaches your preferred level of doneness.
Transfer the steak to a cutting board and let it rest for 5-8 minutes to lock in the juices before slicing against the grain.
Serve the sliced steak alongside the roasted Brussels sprouts and top with freshly sliced avocado.