Place quinoa and vegetable stock in a medium-sized saucepan. Cover and bring to the boil. Allow quinoa to boil for 5 minutes, then turn the heat to low and simmer for 15 minutes. Remove from heat, drain thoroughly and allow to cool to room temperature.
Drain tinned black beans and rinse. Place some paper towels on a rimmed baking sheet and spread beans out to drain completely. Preheat oven to 180°C (350°F) and line a rimmed baking sheet with baking paper.
Place one cup cooled quinoa and half the drained black beans in the bowl of a food processor. Pulse until a chunky paste starts to form. Place paste into a bowl and add cumin, soy sauce, sriracha, pepper, egg, onion, chilli, coriander, quinoa and remaining beans and quinoa flakes and mix thoroughly to combine. Form batter into four patties and press down slightly to flatten.
Bake in preheated oven for 15 minutes. Turn patties over and bake for a further 15 minutes.
Mash avocado with a little salt and pepper and lime juice. Mix sriracha with Greek yoghurt.
To assemble: slice buns and place lettuce on each. Top with a spicy black bean burger, a dollop of mashed avocado and a serving of sriracha yoghurt.