Marinate a 1-pound hanger steak (center sinew removed) in a mix of 1 cup red wine vinegar, 1/3 cup cognac, 1/2 cup olive oil, 1 minced shallot, 2 tablespoons chopped rosemary, 4 chopped garlic cloves, 2 tablespoons sugar, and pepper to taste for up to 6 hours. Preheat a grill to medium-high on one side. Remove the steak from the marinade; pat dry and season with coarse sea salt and pepper. Sear over direct heat until marked, about 4 minutes per side. Transfer to the cooler side; cover and cook until the meat reaches 130 for medium-rare, about 10 minutes. Let rest 5 minutes before slicing. To make the sauce, blanch 1 bunch chives in simmering water for 3 minutes, then rinse under cold water and squeeze dry. Puree with 1 cup sour cream, 3 tablespoons horseradish and a pinch of salt.