1. For the pickled onions, place the vinegar, water, sugar and salt in a bowl and mix well until dissolved. Add the sliced onions and allow to sit for 1 hour. The pickled onion will keep in a jar refrigerated.
  2. Heat a griddle pan over medium- high heat. Brush the bread with olive oil on both sides and toast until golden brown, about 3 minutes for each side.
  3. Spread a generous layer of hummus on 1 side of each toast and top with the tomato, avocado, lettuce and pickled onions. Season with a little salt and pepper between layers.
  4. Secure with a wooden skewer and serve with potato crisps.