Cook the sweet potatoes by baking them in the oven or cooking them in the microwave. To microwave, put the sweet potatoes in a microwave-safe dish, pierce each several times with a sharp knife and cook on high for 4-5 minutes until soft. If you prefer to bake them, preheat oven to 400 degrees. Pierce several times, wrap in foil, and bake for 30 minutes or until softened.
Drain and rinse chickpeas. Add to a large bowl and mash. Spoon the flesh out of the sweet potatoes into the bowl with the chickpeas. Add the cajun spice and garlic powder and mix well. Season with salt and pepper to taste.
Form the mixture into 6 patties, put on a plate and place them in the freezer for 1 hour to firm up (this can be done the night before). Meanwhile preheat the oven to 350 degrees F.
Remove the patties from the freezer and place onto a lined baking sheet. Bake for 30-40 minutes, turning once until firm.
Remove the flesh from the avocado and spoon into a food processor or high powered blender. Add lime juice, Dijon mustard, and garlic powder. 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. Add to the hamburger bun and top with lettuce, tomato and onion slices, and a dollop of avocado mayo. Serve with peaches on the side.