Cook the sweet potatoes either by baking them in the oven or cooking them in the microwave. To microwave them, put the sweet potatoes on a sheet of kitchen paper, pierce each several times with a sharp knife and cook on high for 4-5 minutes until soft.
Spoon the flesh out of the sweet potatoes into a large bowl. Mash the chickpeas and then add to the bowl with the sweet potatoes. Add the cajun spice, mixed herbs and garlic powder and mix well. Season with salt and pepper to taste.
Form the mixture into 4 patties, put on a plate and place them in the freezer for 20-30 minutes to firm up. Meanwhile preheat the oven to 350 degrees F.
Remove the patties from the freezer and place onto a lined baking tray. Bake in the oven for 25 minutes, turning once.
Make the avocado mayo by placing the avocado flesh, lime juice and garlic powder in a food processor. Blitz until everything is well combined and smooth. Slowly drizzle in the olive oil while blending. Season to taste with salt and pepper.
Once the patties are cooked, remove from the oven and build your own burger with a bread bun, lettuce, tomato, onion and a dollop of avocado mayo!