Saute finely chopped onion and celery in butter until soft, add flour, cook stirring over gentle heat for a minute, then slowly add milk, stirring to blend smoothly. Bring to the boil and simmer for a few minutes, stirring constantly to prevent sticking and lumps forming. Add flaked tuna, drained corn, lemon juice and sugar and stir into sauce.

Transfer mixture to a casserole, top with mixed breadcrumbs and grated cheese and cook in 350° oven (moderate) until topping has browned and cheese melted. Serve with fresh crusty bread..