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.
Serve with Roasted Red, White, and Purple Potatoes, which can roast right alongside the steak at 400 degrees (though they'll need to begin roasting before the steak goes in).
nutrition information (per serving):
31, Fat Calories
280, Saturated Fat
43, Monounsaturated Fat
10, Polyunsaturated Fat
Photo: Scott Phillips