In a medium frying pan, heat 2 tsp vegetable oil, add onion, garlic, and mushrooms. Season and fry until everything is soft (approx. 5 min). Put sweet potatoes in a microwavable bowl, cover and microwave on high for 5-6 mins until soft. Mash, and then add to the frying pan. Pour mixture into large mixing bowl and leave to cool.
Add the quinoa, beetroot, egg, coriander, lime zest, flour and some seasoning. Mix with your hands, and then form into four large patties. Sprinkle with flour and chill for 30 mins to firm up.
Put the carrots into a small bowl with the vinegar, sugar, and 1 tsp of salt. Cover and leave to pickle until ready to eat. Mix the mayo with the chili sauce
Fry or grill the burgers to your desired level
Drain the pickled carrots, and toast the buns
Brush the buns with the chili mayo, layer on the spinach, burger patty, and carrot slaw and serve!