1. Simmer potatoes in a large saucepan of water until tender when pierced, 8 to 10 minutes; in last minute, add brussels sprouts, cooking just until they turn bright green. Drain.
2. Preheat broiler. Rub steaks with vinegar, 1 tbsp. oil, and 1 tsp. each salt and pepper. Put steaks on a broiler pan; broil 6 in. from heat, turning once, 8 minutes for medium-rare.
3. Heat remaining 3 tbsp. oil in a 12-in. frying pan over medium-high heat. Add onion and cook, stirring as little as possible, until well browned, 4 to 5 minutes. Add potatoes and brussels sprouts and cook until browned slightly, about 4 minutes. Add wine, scraping up any caramelized bits from bottom of pan. Stir in garlic, rosemary, and remaining 1 tsp. salt and 1/4 tsp. pepper. Serve with sliced meat.