Place the cheese sauce in a medium saucepan and heat gently, then stir in the peas and simmer for 3 minutes. Flake the salmon and remove any skin and bones.
Add the salmon to the pan, heat through and season to taste. Divide the salmon mixture between 2 small ovenproof dishes.
Meanwhile, heat the mashed potatoes in the microwave according to packet instructions. Spoon the hot potato over the top of the fish and sprinkle with the cheese.
Place the dishes on a baking sheet and cook under a preheated hot grill for 3–4 minutes until golden and bubbling. Serve immediately.