Place the quinoa in a saucepan with the stock* and water. Bring to the boil and then reduce the heat to low. Simmer, covered, for approx 15 mins. Remove from heat, stir with a fork and allow to sit covered for 5 mins.
Heat 1/2 tbsp of the oil in a frying pan over a medium heat. Add the onion and cook for around 3 mins, until softened. Add the garlic and the capsicum and cook for a further 3 mins.
Add the quinoa to a mixing bowl. There should be no moisture in the quinoa, if so make sure you drain it really well. Add the onion and capsicum mixture and stir to combine. Add the peas, stir and allow to cool.
Add the tuna (make sure the tuna is drained really well so your mixture does not become too wet) , egg, buckwheat flour and lime zest and stir until combined. Take tablespoons of the mixture and form into cakes.
Place the sesame seeds on a plate and press both sides of each cake into the seeds to coat.
Heat the remaining oil in a frying pan over a medium heat. Cook the tuna cakes for approx 4-5 mins on each side, until golden and cooked through.