Slice the onion as thin as you can - use a mandoline if you want! I've found with the Quick Pickled Red Onion and Melted Brie, there's no need for mustard or ketchup. You may prefer a little spread of mayo or butter on the bread.
Ingredients:1 red onion, very thinly sliced
1/4 cup vinegar (rice vinegar, cider vinegar white wine vinegar or red wine vinegar)
1/2 teaspoon sugar
1 1/2 pounds ground lamb
salt and freshly ground black pepper
8 ounces brie cheese, sliced
4 slices of tomato
4 burger buns
1. 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.
2. 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.
3. Serve the burgers with the quick pickled red onion, buns and tomato.