Skin the fish and cut into 4 pieces. Wash in cold water.
Place in a saucepan with the onion and lemon juice. Add the water and milk. Cover and cook gently for 10 minutes.
Drain the fish, saving the cooking stock.
Preheat an oven to 200°C / 400°F / Gas Mark 6.
Melt the low-fat spread in a saucepan. Add the flour and cook for 2 minutes until it forms a soft ball. Remove from heat and add fish stock gradually, stirring continuously. Bring to the boil, reduce heat and cook for another 2 minutes, still stirring.
Grease a casserole dish and add a little sauce. Place fish in the dish and cover with the salt and pepper, remaining sauce, peas and carrots.
Spoon the mashed potatoes over the filling.
Place the casserole dish in the preheated oven and cook for 20 minutes or until piping hot and browned on top.