Beef Tenderloin with Pickled Onions and Pink Peppercorns

Preheat the oven to 350°. In a large bowl, stir the vinegar with 1/4 cup of the sugar and 1 tablespoon of salt until the sugar and salt are dissolved. Add the onions and let stand at room temperature, stirring occasionally, until softened, about 1 hour. Drain the onions, reserving the pickling liquid. Meanwhile, rub the beef with the oil and set it in a small roasting pan. Season with salt and pat with 1 tablespoon of the peppercorns. Roast on the lowest rack of the oven for about 40 minutes, until an instant-read thermometer inserted in the center registers 125° for medium-rare. Transfer the roast to a board and let rest for 30 minutes. In a saucepan, combine the rosé with the remaining 2 tablespoons of sugar and boil over high heat until reduced to 1/2 cup, about 20 minutes. Add 1/4 cup of the pickling liquid and bring to a simmer. Add the cornstarch slurry and the remaining peppercorns and cook until thickened, about 1 minute. Slice the meat, removing the strings. Transfer to plates and top with the onions. Drizzle with the sauce and serve.