Step two
While the potatoes are boiling, preheat the oven to 200C (fan 180C, gas mark 6). Put the remaining milk, low-fat spread and flour into a saucepan. Bring to the boil over a medium heat, stirring continuously with a small whisk or wooden spoon until the sauce bubbles and thickens. Stir in the parsley (if using) and the peas, and season with pepper. Turn off the heat.