Heat oil in a pan and cook celery and red pepper for 2 minutes. Drain and cool.
Put diced apple into a bowl with the lemon juice to stop the apple from going brown.
Mix mushrooms, cheese and basil together in another bowl.
Lift apple out of lemon juice and drain. Stir apple into mushroom mixture with the cooled celery and red pepper.
Brush a sheet of filo pastry with a little of the melted butter then fold in half to give a rectangle 23x28cm (9×11”). Brush the top of the pastry with more butter, and then sprinkle some breadcrumbs over the top.
Place a quarter of the mushroom mixture down two-thirds of the pastry. Starting at the shorter edge, and at the end where the mushroom mixture is, carefully roll up pastry to form a sausage shape. Place on a lightly greased baking sheet.
Repeat with the remaining pastry and mushroom mixture to make four strudels. Brush strudels with melted butter and sprinkle remaining breadcrumbs over the top. Bake in the oven for 30 minutes.