Rinse the tomatoes, rub dry and cut across. Remove the inside with a small spoon. Drain the tomatoes upside down.
For the filling, cook the couscous in salted water for about seven minutes according to the instructions on the packet. Drain into a sieve and allow to cool.
Finely dice the feta. Clean the spring onions, rinse and cut into fine rings. Slice olives. Rinse the parsley, shake dry and finely chop. Mix the couscous, feta, spring onions, olives, parsley and yoghurt together. Season to taste with salt and pepper.
Season the inside of the tomato halves with salt and pepper, pour in the coucous mixture. Serve on a bed of lettuce leaves or flat leaf parsley.