1. Cook pasta in salted water until al dente.
2. Peel the cucumber, half lengthwise and scrape out the seeds. Cut into slices and saute in butter. Add stock and milk and simmer for 5min.
3. Dissolve cornstarch in some water. To thicken the sauce, bring it to the boil and gradually add the cornstarch while stirring, stop when it has reached the desired consistency. Turn off the heat and add tuna to the sauce.
4. Season to taste with dill, salt and pepper.