If you are using chicken thighs, you can probably skip the brining. But if you are using chicken breast, I recommend brining in lukewarm salted water for about 15 minutes before roasting. Basically, stick your chicken in a bowl of very salty water, leave it to sit for 15 minutes, then rinse and pat dry.
Preheat the oven to 230 C (about 450 F).
Stick your brined (or not) chicken breast in a large casserole dish and smear the delicious butter on top. Stick this in the very hot oven for about 20 minutes while you prepare the rest.
Break up the cauliflower into small bits and put into a baking pan and sprinkle salt and the olive oil over the top. Set aside.
Chop up your chorizo into little cubes. Cut your leek into small bits. Set aside half of one of the leeks!
Start cooking the chorizo in a skillet and, after about 5 minutes, throw the leeks in (except for the half you've set aside!). Let this cook for a while until the leeks have softened.
Your chicken is probably done by now (depending on how quickly you chop!) so take that out of the oven and put the cauliflower in. Turn the temperature down to about 180. Set the timer for about 30 minutes.
Let the chicken cool a bit (do as I say, not as I did, ouch.). Then chop or shred it and add it to the skillet with your chorizo and leeks. Turn the heat down a bit to let it simmer.
When all is combined, stir in the cream cheese until it melts and everything is nicely mixed. Switch off the hob and wait for the cauliflower to cook and wonder why my timing isn't better.
When the cauliflower is tender, take it out of the oven and, using a traditional potato masher, mash it right in the tray. You could make it smooth, but I personally like the texture. Once it's mashed (you don't need to add any butter as there should be plenty of olive oil in the pan), throw in the mustard and the half a leek (chopped!) that you kept aside from earlier.
Now spoon the chicken, leek and chorizo mixture back into the casserole dish. Top with the cauliflower mash. Then grate the cheddar over the top.
Throw the casserole back into the oven and bake for 20 minutes until the cheese is bubbling.