Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Peel the kohlrabi and cut it into 1-inch cubes, then toss them on the baking sheet with 0.5 tbsp olive oil, garlic powder, 0.25 tsp salt, and 0.125 tsp pepper.
Roast the kohlrabi for 20-25 minutes, tossing halfway through, until the edges are golden and tender.
While the kohlrabi roasts, pat the sirloin steak dry with paper towels and season both sides with the remaining salt and pepper.
Heat the remaining 0.5 tbsp olive oil in a heavy-bottomed skillet or cast iron pan over medium-high heat until it begins to shimmer.
Place the steak in the hot pan and sear for 3-4 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the pan and let it rest on a cutting board for 5 minutes to ensure the juices redistribute.
Slice the steak against the grain and serve immediately alongside the roasted kohlrabi, garnished with fresh thyme leaves.