Melt the butter in your saucepan over a gentle medium-low heat. Add the finely diced onions and cook them slowly until they are soft and translucent.

Make sure the onions do not turn brown or crispy during this stage. You want them to stay pale and sweet for a delicate white sauce.

Stir the plain flour into the buttery onions to create a thick paste. Cook this for one to two minutes to remove the raw flour taste.

Slowly add the milk to the pan, just a little bit at a time. Whisk or stir constantly to keep the sauce smooth and free of lumps.

Bring the mixture to a very gentle simmer over medium heat. Continue stirring until the sauce coats the back of your wooden spoon nicely.

Add the salt, black pepper, and ground nutmeg to the pan. Nutmeg is a secret ingredient that makes the onion flavor really stand out.

Stir in the English mustard if you want a tiny bit of heat. This helps cut through the richness of the butter and whole milk.

Pour in the tablespoon of heavy cream at the very end. Stir it through until the sauce looks perfectly glossy and very inviting.

If the sauce is too thick, add a small splash of extra milk. It should pour easily but still look thick and rich.

Pour the hot sauce into a warmed gravy boat or jug. Serve it immediately while it is still steaming and full of flavor.