Heat the coconut oil in a cast-iron skillet, add the garlic, fry for a few seconds then add the spinach. Cook for 1-2 minutes until it softens, then transfer into a bowl.
In the same bowl add quinoa, egg, onion, salt and freshly ground black pepper and mix to combine.
Grease the skillet with coconut oil, shape the burgers using your palms and fry them for 3-4 minutes on each side.
Place the avocado, parsley and lime juice in the blender and process to obtain a smooth sauce.
Assemble the burgers: cut the buns in halves, add avocado sauce on the bottom half, add the quinoa burger, top with onion rings and more avocado sauce, close the burger with the bun top and serve right away.