Slice onion into strips, tomato into slices and mash avocado in a small bowl.
Slice open burger bun and toast insides on a pan on medium heat. At the same time, mix sauce ingredients in a small bowl until combined.
Remove buns once toasted and sauté onions in half the olive oil.
Push onions to one side of the pan and crack the egg on the other side in remaining oil.
Season egg with cajun seasoning, sea salt and cook to your liking. I prefer the egg yolk to be runny so I don't break the yolk and cook it for approx. 3 minutes.
Whilst egg is cooking, spread avocado on the bottom of the burger bun, lay lettuce on top followed by the tomato. Season tomato with sea salt.
Add cooked onions on top of the tomato followed by the cooked egg.
Add sauce to the top of the burger bun and place on top to close the burger. Cut in half and dig in!