Position a rack in the center of the oven and heat the oven to 400°F. Trim and cut the steak into 4 portions. Season the steaks all over with the rosemary, 2 tsp. salt, and 1/2 tsp. pepper.
Heat 1 Tbs. of the oil in a large (12-inch) skillet over medium-high heat. Working in 2 batches if necessary, arrange the steaks in the skillet in a single layer and cook, turning once, until nicely browned, 3 to 4 minutes per side. Remove the skillet from the heat, transfer the steaks to a rimmed baking sheet, and roast until medium rare (130°F to 135°F), 4 to 6 minutes more. Set the steaks aside to rest.
Meanwhile, return the skillet to medium-high heat. Carefully add the wine and cook, scraping up any browned bits with a wooden spoon, until reduced by about half, 3 to 4 minutes.
Add the garlic to the skillet and cook until fragrant, about 10 seconds. Whisk in the vinegar, sugar, mustard, 1/4 tsp. salt, and 1/8 tsp. pepper. Drizzle in the remaining 3 Tbs. oil while whisking constantly.
Add the chard stems and cook, stirring occasionally, until beginning to soften, 5 minutes. Add the chard leaves in batches and cook, tossing, until the leaves are wilted enough to fit comfortably in the skillet, about 2 minutes. Cover the skillet and cook, tossing once or twice, until just tender, about 5 minutes.
Transfer the steaks to plates and top with the chard. Sprinkle with the Pecorino Romano, if using, and serve.