Leeks In Cheese Sauce

Immediately, I'm sure many of you have come across this title and dismissed it as retro-mush. And fair enough, too. This kind of dish is very old-fashioned, but it's a shame terms such as 'retro', 'vintage' or 'old-fashioned' connote fashion, and I am the quintessential person who pushes against the current of what's cool at the moment. In food, it can be the best thing to invite dormant recipes back to the table; alright, old recipes that worked on the principle that everything is better suspended in jelly are perhaps better off fossilised, but things like this are perfect for revitalising.
It's very simple, to get a delicious cheese sauce, use delicious cheese. I love some Swiss in there, plus blue cheese and a bit of soft cheese (or cut out the middle man and use soft blue cheese like dolcelatte), mozzarella is lovely for stringiness, but bear in mind it doesn't have a lot of taste. For budget, I bulk out these more expensive cheeses with a cheap but good sharp cheddar or red Leicester  (which helps for colour too) and some very hard cheese like parmesan. I have to say, I used to make white sauces where the roux was an insoluble thick ball of dough, like pastry that had to be diluted with milk, and the hateful job seemed to never end. I don't bother with that now, make the roux runny enough so it blends with the milk easily.
I made this as part of the main in a three course dinner party, but there's nothing to stop you having this as a day-to-day meal.

Take a nice pan that will go to the oven (think cast iron) and brown the leeks until soft and well coloured. Continue to cook until they're al dente soft. Remove the leeks to a plate and tip away any excess liquid before continuing.
Melt 80g butter into the caramel scorched pan and add 3 tbsp flour and 2 tso mustard powder. Stir until you have a smooth paste. Turn off the heat and add 1 litre full fat milk, stirring constantly to avoid lump stress. You don't need to heat the milk up so much here, as the dish needs to be wide and shallow so has a big surface area to heat the sauce up to thicken. Add approx. 250g grated cheese and stir off the heat until just melted. Season to taste with salt, pepper and nutmeg.
Return the leeks to the sauce and top with breadcrumbs and a little more grated cheddar cheese.
Bake in a hot oven until bubbling and golden, which only takes around 10 minutes as the leeks are already cooked.
I should tell you that this makes fabulous cauliflower cheese- steam florets of 1 head cauliflower for 6 minutes until slightly soft and line these over a baking dish. Pour over the cheese sauce made separately in a saucepan and crumble a bit more cheese on top. Add 2 tbsp breadcrumbs and bake for 15 minutes until bubbling and the cauliflower is hot all through. If you want to skip the steaming part, the cauliflower will go in raw but you will have a runnier sauce and you will need to bake it for about 35 minutes. This is fine, it's not as if you have to do anything in at the time. That is if you're not making hot fresh chips to go with this.