Make the dough following pack instructions, reserving a few tbsp of the mix for dusting. Heat 2 tsp of the garlic butter in a large frying pan, then cook the aubergine rounds, in batches, for 1-2 mins each side, until golden. Remove and season, then set aside. Add a little more butter and fry the mushrooms until soft, then remove and set aside.
Lightly butter an 18cm round cake tin, about 4cm deep. Dust a flat surface with the reserved bread mix and roll out two-thirds of the dough to come over the sides of the tin. Line the tin and press the dough into the corners. Leave the sides hanging over.
Layer up the ingredients, starting with half the aubergine, basil, pasta sauce, mozzarella and mushrooms. Repeat with the remaining ingredients, pushing down well. Roll out the remaining dough to form a lid and put it on top. Pinch together the lid and the sides, twisting them together to form a crusty edge. Wrap well and freeze. Defrost at room temperature for about 8 hrs.
Heat oven to 200C/180C fan/gas 6. Cook pie for 30 mins. Remove from the tin and place on a baking sheet. Brush with garlic butter, then cook for 20 mins more, brushing again with butter before serving.