Another sauce which starts off life as a roux, Mornay Sauce is a onion and cheese sauce that goes well with fish. It's also commonly used in French breakfast recipes with eggs. It's strong in flavour, where there's definitely creative license to how much cheese you add in the final step.
- 150g salted butter
- 50g plain flour
- 1 pint whole milk
- 2 leeks
- 100g mature cheddar cheese
- Slice the leeks finely and fry on a low heat with 50g of butter, once cooked through put to one side
- Melt the remaining butter on a low heat in a pan
- Add the flour and mix with the butter until you have a loose paste
- Ensure you cook off the paste for a short while so the sauce doesn't taste like raw flour
- Add the milk bit by bit, keeping it moving with a whisk. It will go through phases of being thick and loose, and as you introduce more milk and heat it will thicken up
- It's important to keep the sauce moving to ensure no lumps form or the sauce starts to burn on the pan
- Once the roux is finished, add the cheese and leeks, then season (to ensure it's not too salty from adding the cheese beforehand)
The recipe cited below notes to add a clove infused onion and some bay leaves. I opted to change the onion for leeks in the sauce for a bit more texture.
There's a nice run-down of the classic sauce recipe on The Spruce Eats. There's also a nice primer on the 5 mother sauces, which the base of this one (béchamel) is one of.