For the wellington
1 1/2 tbsp olive oil, plus a little extra for brushing
200 g small chestnut mushrooms, quartered and sliced
6 spring onions, trimmed, halved lengthways and thinly sliced
2 cloves garlic, finely chopped
2 tsp fresh lemon juice
2 tsp chopped tarragon
2 tbsp half-fat creme fraiche
12 sheets filo pastry
4 x 150 g skinned wild salmon fillets
For the dressing
3 tbsp finely chopped watercress, leaves, plus extra whole small sprigs to garnish
2 tsp finely chopped tarragon
2 tsp finely snipped chives
2 tbsp olive oil
2 tsp small capers, drained
lemon juice, optional, to taste
For the salmon wellingtons: preheat the oven to 200C/fan 180C/gas 6.
Heat the oil in a large non-stick frying pan. Tip in the mushrooms, spring onions and garlic and stir-fry over a high heat for 4-5 minutes or until the mushrooms are just starting to colour and all the liquid from them has evaporated.
Splash in the lemon juice and let that evaporate too. Remove from the heat, stir in the tarragon, season and leave to cool. When cool, stir in the creme fraiche.
Trim the filo sheets with scissors so each is 28 cm square.
For each wellington,lay one of the filo squares on the work surface and brush with a little oil. Lay two more filo squares on top, brushing in between each with oil.
Lay a salmon fillet in the centre and season it with salt and pepper. Spread a quarter of the cooled mushroom mix over the top of the fillet.
Carefully bring the pastry edges up into the centre to enclose the salmon, scrunching it together and lightly pressing it at the top to seal and form a pouch.
Place the parcels on a lightly oiled baking sheet, brush the outsides with a little more oil and bake for 20 minutes to cook the fish and until the pastry is crisp and golden.
If the tops are browning too quickly, cover loosely with foil for the last 5 minutes of cooking time.
For the watercress-herb dressing: meanwhile, mix together the chopped watercress, tarragon and chives, then stir in the oil, vinegar and capers, adding salt and pepper and lemon juice, if needed, to taste.
Once cooked, let the salmon wellingtons sit for a couple of minutes, then serve with a drizzle of the dressing, garnished with small watercress sprigs.