- Preheat the oven to 180°C/gas 4. Heat the oil in a saucepan and fry the onions for 4-5 minutes until soft, but not brown. Add the leek and cook for a further 3-4 minutes.
- Sprinkle the flour over the onions and leek. Mix well. Add the mustard and mix again.
- Gradually stir in the milk and keep stirring for 2-3 minutes until the sauce thickens.
- Add three-quarters of the Cheddar and stir until it has dissolved, mix in the peas and remove from the heat.
- Stir the macaroni into the sauce, pour into an ovenproof dish and sprinkle with the remaining Cheddar. Bake for 20 minutes.
- You could use wholemeal pasta for added fibre – if you can't find it in the supermarket it's available in health shops. But wholemeal penne or any pasta shape would work, too.
- You could add sweet corn or cooked mushrooms, cauliflower or broccoli.
- Freezing instructions: Suitable for freezing once cooked. Defrost in the fridge or microwave and reheat until piping hot throughout.