Place tuna, cucumber, feta, red bell pepper, red onion, mint, parsley, olives radish and the lemon zest (not the juice) in a large mixing bowl and mix to combine.
In a smaller mixing bowl, pour in the olive oil, Dijon mustard, juice from the two lemons, garlic, salt, pepper and red pepper, if using.
Use a stick blender or whisk to combine thoroughly.
Pour the dressing into the tuna mixture and stir to combine.