Preheat the grill, and oven to 200ºC/Fan 180ºC/Gas 6 and then line a loaf tin with foil and oil lightly.
Slice the aubergine thinly, lengthways. Arrange on a large oiled baking tray in a single layer, brush or drizzle with olive oil and season with salt and pepper. Cook under the hot grill for 5-7 mins each side until the aubergine has softened and is beginning to turn golden. The aubergine will cook to a deep brown once in the oven, so don’t worry about getting too much colour at this stage. Once you have turned the aubergine slices over, do keep a close eye on them as the second side will colour more quickly than the first side. Allow to cool slightly.
Use the aubergines to line across the base and sides of the prepared loaf tin, all slices going in the same direction.
Melt the butter in a medium pan, add the onion, celery and garlic and cook, stirring occasionally, untill soft, about 10 mins. Spoon into a large bowl and leave to cool.
Add the remaining ingredients to the bowl with plenty of seasoning, and stir well to mix.
Spoon into the loaf tin, pressing the mixture down firmly. Fold the ends of aubergine over the top of the filling and cover the tin with the foil.
Cook in the preheated oven for 50 minutes to 1 hour. Turn out onto a serving plate, remove the tin and garnish with sprigs of flat-leaf parsley.