Place sesame seeds and salt in a small frying pan; cook, stirring, over medium heat until seeds are a deep golden brown and aromatic. Remove from heat; cool. Transfer to a small food processor or mortar and pestle. Process until finely ground; transfer to a small bowl. 
Wipe out the food processor bowl.

Cut tomatoes into quarters; scoop 
out seeds with a teaspoon. Place half the tomato seeds into food processor; add lemon juice and oil. Process to a smooth puree; season to taste with sea salt flakes and freshly ground black pepper.

Cut the remaining tomato flesh into 5mm pieces. Cut the cucumber and radishes to a similar size.

Combine tomato, cucumber, radish and onion in a serving bowl. Add the tomato puree and season to taste with some of the sesame salt. Toss to combine, then gently fold through the herbs. Crumble the fetta over the top.