Recipe kindly shared by the Tesco Community Cookery School with Jamie Oliver. The recipe has been written to be flexible and scalable up to 50 portions. Note: all veg weights are for trimmed, prepped vegetables.
Peel and roughly chop the onions.
Trim, wash and slice the leeks.
Place a pan large enough to hold all the ingredients on a medium heat with the oil and butter.
Add the vegetables and cook with the lid on for 30 minutes or until the vegetables are softened and lightly coloured, stirring every so often.
Tip in the flour and stir it into the buttery veg until well mixed.
Divide the milk between three cups and add one cup to the pan, stirring until completely incorporated. Add the second cup and repeat.
Pour in the remaining milk and bring to the boil, stirring continuously.
Reduce the heat to low and cook for 20 minutes or until thickened, stirring regularly.
Carefully pour the sauce into a clean pan, leaving behind any burnt bits that are stuck to the bottom, and whiz with a stick blender until smooth. Season to taste with salt and pepper.
Let your sauce cool as quickly as possible, then cover, label and chill, or freeze.