Heat some olive oil in a frying pan and cook the red onion for about 3-4 minutes until softened.
Add the chopped garlic to the pan and cook for 1 minute more.
Meanwhile, put all of the burger ingredients in to a large mixing bowl and stir together. You can then either blitz the mixture in a blender for a smooth consistency or, if you prefer a chunkier texture, mash it with a potato masher.
Divide the mixture in to 4 (or however many you need, the rest will keep in the fridge and also freezes well.) Using your hands roll them in to balls, then squash in to patties of about 1cm thickness. Tip: Have a bowl of flour next to you to dip your hands in to between rolling each patty, this will prevent them sticking.
Cover and place in to the fridge for at least 15 minutes.
When you're ready to cook the burgers, heat some olive oil in a large frying pan. When it's hot place the burgers in and cook on one side for 5 minutes.
Carefully flip the burgers over, then drape a slice of cheese over the top. Cook for a further 5 minutes, with a lid on to help the cheese melt.
Serve on the warmed burger buns, with burger sauce, sliced avocado, cherry tomatoes, salsa and pickles.
