Preheat the oven to 200C/180C Fan/Gas 6.
Lightly butter a large sheet of foil and sit the salmon on top, then season with salt and freshly ground pepper and wrap in the foil to make a parcel. Put the foil parcel on a baking tray and bake in the oven for 15–20 minutes, or until the salmon is cooked through. Set aside to cool.
Meanwhile, cook the rice until tender in salted boiling water according to the packet instructions. Drain and tip into a bowl.
Melt 25g/1oz of the butter in a large frying pan over a medium heat. Add the onions and fry for 3 minutes, with a lid on the pan, until starting to soften. Add the garlic and chilli and fry for 30 seconds, then add the mushrooms and fry for another 3 minutes.
Stir in the cooked rice and remove from the heat before adding the lemon zest, juice, parsley and chives. Break the cooked salmon into large pieces and add to the rice. Season with salt and freshly ground black pepper and carefully stir to combine. Set aside to cool and then chill in the fridge, covered, for 30 minutes.
Melt the remaining butter in a small pan. On a piece of baking paper, lay 2 sheets of filo pastry lengthways and side by side – they should overlap slightly (by about 5cm/2in). Brush with the melted butter and seal along the middle to make a 45cm/18in square. Lay 2 more filo sheets side by side on top, at right angles to the first layer, and brush with butter. Lay the last 2 sheets over the top, in the same direction as the first 2 sheets, and brush with butter to make 3 layers forming a 45cm/18in square.
Arrange the salmon and rice filling along one-third of the square in a long log shape, leaving a gap of at least 5cm/2in from the edge of the pastry. Fold in the sides and then tightly roll up the pastry around the filling to create a strudel shape about 30cm/12in long. Use the baking paper to help to roll it up firmly. Make sure the ends of the pastry are sealed and then transfer the parcel with the paper to a baking tray.
Brush more melted butter over the top and then bake at 200C/180C Fan/Gas 6 for 30–35 minutes, or until the pastry is crisp and golden-brown.
For the sauce, add all the ingredients to a bowl, season with salt and freshly ground black pepper and stir to combine. Slice the cooked coulibiac and serve with sprigs of watercress (if using) and the sauce.