1: Make the sauce: Melt the butter in a small saucepan on your grill’s side burner or on the stove. Add the onions and caramelize over medium heat (cook until soft and browned), 5 to 8 minutes, stirring often. Reduce the heat as needed so the onions soften and brown without burning. ShareTweetPin193 Shares 2: Whisk in the cream and boil until reduced by half. Whisk in the mustard and add salt and pepper to taste: the sauce should be highly seasoned. Keep the sauce warm. It can be made up to several hours ahead. ShareTweetPin193 Shares 3: Brush the hanger steaks on all sides with mustard, then season generously with salt and pepper. ShareTweetPin193 Shares 4: Set up your grill for direct grilling and preheat to high (450 degrees). Brush or scrape and oil the grill grate.ShareTweetPin193 Shares 5: Arrange the hanger steaks on the diagonal on the grill grate. Grill until sizzling and browned on the outside and cook to taste, about 2 minutes for rare (120 degrees); 3 minutes per side for medium-rare (130 degrees). Give each steak a quarter turn after 1 minute to lay on a crosshatch of grill marks, if desired. Use an instant-read thermometer inserted through the side of the steak to check for doneness. Alternatively, use the poke test. ShareTweetPin193 Shares 6: Transfer the steaks to a cutting board and let rest for 2 minutes. Meanwhile, reheat the mustard sauce. Cut the hanger steak on the diagonal into 1/2 inch slices and transfer o a platter or plates. Pour the mustard sauce on top; sprinkle with chives; and dig in. ShareTweetPin193 Shares