Preheat the oven to 200ºC, 180ºC fan, gas mark 6. Line a baking tray with baking parchment and arrange the salmon fillets on top.
Mix the soft cheese and the chopped chives together in a small bowl, then spread the mixture over the top of the salmon fillets.
In a separate bowl, loosen the mashed potato with a fork, then spoon onto the salmon fillets. Brush with the melted butter.
Bake for 12-15 minutes until the salmon is cooked through and the potato is crispy on top.
Meanwhile, cook the peas in a pan of boiling water for 3-4 minutes. Drain and serve with the salmon and rocket.