Wash the quinoa and cook in boiling salted water for about 15 minutes
Wash, clean, seed and dice the tomatoes and cucumber. Peel onion and cut into thin rings. Cut olives into fine rings, drain and pluck the tuna
Drain the quinoa, rinse with cold water and drain. Season the vinegar with salt, pepper and sugar, fold in the oil. Put the quinoa, tuna, tomatoes, cucumber, onion and olives in a bowl, pour the vinaigrette over it and mix everything. Crumble the feta and garnish with oregano