For serving, the usual mustard and ketchup work fine, but I prefer these with some sautéed onions and a sweet Dijon sauce. To create the sauce, in a small bowl, mix together the mustard and 3 tablespoons syrup (or more, depending on how sweet you like your condiments) and set aside. In a sauté pan, sauté the sliced onion in a little oil over medium-high heat until browned. Spread burger buns with vegan mayo, place a hot burger patty on the bun, and top with a generous helping of the onions and sweet Dijon sauce. If you like, throw on some lettuce and tomatoes, too.