Preheat the oven to gas 4, 180°C, fan 160°C. Mix the honey, mustard, 1 tbsp lemon juice and zest to form a paste, then brush over the salmon fillets.
Put the fish fillets in the oven for about 15 minutes or until cooked through and golden on top. Blanch the watercress for at least 15 seconds in boiling water and allow to cool. Using a fork, gently flake the roasted salmon into chunks, then combine with the watercress, avocado, chopped gherkin and mango in a salad bowl.
To make the dressing, mix together the lemon juice, olive oil, white wine vinegar and sugar. Season to taste, stir again, then drizzle it over the salad.
Toss the ingredients so that they are well coated in the dressing, then serve.
For more salmon inspiration, see Salmon recipes.