Cook potatoes in boiling salted water until tender, drain and then cool. Brush the salmon fillets with olive oil and season with pepper before searing in a hot pan for 2 mins on each side. Test salmon ensuring it is still springy to touch. Allow to cool to room temperature before breaking the salmon into bite-sized pieces. Combine the salmon, potatoes, watercress, rocket and thinly sliced spanish onion in a large bowl.
To make the dressing combine the buttermilk, mayonnaise, garlic, mustard, dill and lemon. Whisk until smooth and season to taste.