Add all ingredients to a glass jar and shake to combine. Set aside.
Add 1/3 cup of quinoa and 2/3 cup of water to a small saucepan and bring just to the boil on high heat, covered. Once boiling, turn down to low and continue to simmer covered, for around 15 minutes, or until all the water is absorbed. Remove saucepan from the heat and fluff quinoa with a fork then set aside to cool a bit.
Add the quinoa, tuna, cherry tomatoes, radish and herbs to two plates and crumble over the feta cheese. Drizzle over the dressing and season further if you like.