Dressing - Put all the ingredients EXCEPT the water in a jug just large enough to fit the head of a stick blender. Blitz until smooth. Add water 1 tsp at a time until it's a thick drizzle-able consistency (don't dilute flavour too much). Then taste and add more salt if needed (feta saltiness varies by brand).
Salad - Place all the salad ingredients EXCEPT the feta and tuna in a large bowl. Pour over the dressing, toss. Crumble feta in, toss (it will smear - we like!). Add the tuna and gently toss, trying to keep some chunks rather than it all crumbling into tiny pieces.
Taste and add more lemon if desired (I enjoy this on the tart side - tuna can take it!).
Serve in bowls with spoons for digging in! Tuck warm pita pockets on the side, or use the tuna salad to stuff inside the pita (it's sooooo good!).