Mix the celery salt with a little pepper and rub all over the tuna. Cut the fennel bulb in half, then into thin slices.
Heat the oil in a flameproof casserole and fry the tuna on all sides until browned. Drain. Add the sliced fennel to the pan and fry gently until softened.
Add the lentils and wine and bring to the boil. Boil until the wine has reduced by about half. Stir in the stock, fennel leaves or dill, capers and tomatoes and return to the boil. Cover with a lid and transfer to a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes.
Return the tuna to the casserole and cook gently for a further 20 minutes until the lentils are completely tender. The tuna should still be slightly pink in the centre. If you prefer it well done, return to the oven for a further 15–20 minutes. Check the seasoning and serve.