Preheat your oven to 425°F and prepare a large rimmed baking sheet.
Halve the cherry tomatoes and thinly slice the red onion into wedges.
Toss the tomatoes and onions on the baking sheet with 0.5 tbsp olive oil, a pinch of salt, and pepper.
Roast the vegetables for 12 minutes until the tomatoes begin to burst and release their juices.
While vegetables roast, pat the skirt steak dry and season both sides with garlic powder, rosemary, salt, and pepper.
Heat the remaining 0.5 tbsp olive oil in a cast-iron skillet over high heat until shimmering.
Sear the steak for 2 minutes per side to develop a deep brown crust, then transfer the skillet to the oven.
Roast the steak for an additional 4-5 minutes alongside the veggies until it reaches your desired doneness.
Remove from the oven, drizzle the entire dish with balsamic vinegar, and let the steak rest for 5 minutes before slicing against the grain.