Let the mixture simmer gently without a lid on the pan, stirring from time to time, until the liquid has disappeared and it is quite thick - this will take about 20 minutes.
Add the mushrooms and cook for a further 5 minutes or so, or until the mushrooms are tender and any liquid which they make has boiled away.
Remove from the heat and season to taste.
When the filling mixture has cooled, you can assemble the strudel, which can then be cooked straight away, or kept in the fridge for a few hours until you"re ready, or in the freezer for at least a month.
I find it easiest to assemble the studel directly on to a large baking sheet, which means you don't have to lift it later.
Set the oven to 200°C / 400°F /gas mark 6 if you have the type of filo pastry which is long and narrow — about 30 x 20 cm (12 x 8 inches) — lay two sheets side by side, overlapping them slightly where they join.
If you have the type which is quite large — about 30 cm (12 inches) or so square — lay out one sheet.
Either way, brush the surface with a little melted butter and then sprinkle with a third of the nuts.
Put another layer of filo pastry on top, brush with butter, and scatter with nuts.
Repeat with another layer, then a final layer of filo pastry, which you just brush with butter.
Now tip the tomato filling mixture on top and spread it to about 2.5 cm (1 inch) of the edges.
Fold the edges over, to enclose the edge of the filling then, starting from one of the widest edges, roll the whole thing up like a Swiss roll, trying not to let it break (although it's not the end of the world if it does).
Brush with melted butter and garnish with some shreds or shapes of filo pastry.
Put the strudel into the oven and bake for 30 minutes, or until golden brown.
Transfer carefully to a serving dish with the aid of two slices.