Preheat the oven to 200°C, 180°C fan, whizz the sundried tomatoes in the small bowl of a food processor to a paste with 1 tbsp of the oil from the jar. Stir in the mustard seeds and press on to the skinned side of the salmon fillets. Line a baking sheet with foil and drizzle with the oil. Place the salmon on top and bake in the oven for 10 – 15 minutes. Leave to cool completely then slice each piece into 3, carefully holding the topping in place as you slice.
Share the potato salad between 6 serving plates. Squeeze over a little lemon juice and season to taste. Scatter over the watercress and lay 2 slices of salmon on top of each plate.