Peel and dice the tomatoes. Peel the cucumber, halve lengthwise and scoop out the seeds. Slice cucumber.
Heat the olive oil in a saucepan. Saute onion for 2 minutes, then add cucumbers, tomatoes, water, vegetable stock, tomato paste and half the dill. Simmer 15 minutes then puree the soup (add more water if necessary).
Serve garnished with a spoon of yoghurt and remaining dill.