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In a medium frying pan, heat 2 tsp vegetable oil. Add the onion, garlic and mushrooms, season and fry until everything is soft, about 5 mins. Put the sweet potatoes in a microwaveable bowl, cover with cling film and microwave on high for 5-6 mins until soft. Mash the sweet potatoes, then add to the pan with the onion mix. Tip into a large mixing bowl and leave to cool.
Add the quinoa, beetroot, egg, coriander, lime zest, flour and some seasoning. Mix with your hands, then form into four large burgers. Sprinkle with flour and chill for 30 mins to firm up.
Put the carrots in a small bowl with the vinegar, sugar and 1 tsp salt. Cover and leave to pickle until ready to eat. Mix the mayonnaise with the chilli sauce.
Heat a frying pan and pour in a thin coating of oil. Fry the burgers on a medium-low heat to allow the centre to cook slowly. When browned, after about 7 mins, gently flip over and cook for the same time on the other side. Drain the pickled carrots and toast the buns.
Brush the buns with the chilli mayo. Layer on the spinach or rocket, burgers and the carrot slaw.