Preheat the oven to 200C/400F/Gas 6. Grease a baking tray with melted butter.
Heat the oil in a heavy-based frying pan and gently fry the onion for five minutes, or until softened. Remove the onion and place into a large bowl.
Melt 150g/5oz of the butter in the frying pan until foaming. Then add the mushrooms, garlic and thyme leaves and cook for five minutes, or until the mushrooms have softened.
Remove from the heat and add the mushrooms to the onion. Add the artichoke hearts and pine nuts to the bowl, and season with salt and freshly ground black pepper. Mix until well combined. Allow to cool and then drain the mixture through a sieve.
To assemble the strudel, melt the remaining butter and place into a small bowl. Lay out two sheets of filo pastry, overlapping at the thin end, to cover the greased baking tray. Brush the pastry with some of the melted butter.
Cover the pastry with a further two layers and brush with melted butter.
Add a final layer of pastry, but only brush the edges of the pastry with melted butter. Place the mushroom mixture onto the unbuttered pastry and roll up to form a parcel, tucking the sides in.
Brush well with more melted butter and bake for 25 minutes, or until crisp and golden-brown.