In a pan, heat the olive oil until it sizzles. To this, add your sliced onions and saute for approximately 17 mins on a medium heat. Stir regularly to ensure they don't burn. If the pan dries out, add a dash of water each time until the onions have caramelised into a beautiful golden brown.
Add salt, pepper and the balsamic vinegar and cook for a further 2-3 mins to infuse the caramelized onions. Remove from heat.
In a bowl, mash your avocado and add oregano and salt and pepper to taste. Then fork this onto your slices of toast. Top with your caramelized onions. Enjoy!