Fill a small bowl with the vinegar, sugar plus 1/4 cup of water. Add in the red onion slices and let sit at room temperature while you complete rest of recipe. Season the ground lamb with salt and pepper. Form into 4 equal sized patties. Heat a grill pan over high heat. When hot, add in the patties. Grill for 2 minutes, then flip the patties. Turn the heat to medium and let cook for 5 minutes. Flip again and cook an additional 3-5 minutes, depending on doneness preference. During the last 30 seconds of cooking, add the brie cheese slices on top of each burger. Serve the burgers with the quick pickled red onion, buns and tomato.
