Ingredients
1kg potatoes, I like Vivaldis, peeled and cut into chunks
125g butter
1 tbsp olive oil
100g brussels sprouts or cabbage, sliced
4 spring onions, finely sliced
2 shallots, finely sliced
1 small leek, finely sliced
50g plain flour
350ml full-fat milk
300ml double cream
50g Cheddar cheese, grated
5 tbsp fresh flat leaf parsley, finely chopped
a good grating of fresh nutmeg
450g leftover cooked gammon or ham, cut into chunks
salt and black pepper

Equipment Needed
Potato masher
2 litre oven dish

Method
Heat the oven to 200c.

Put the potatoes in a pan of cold, salted water and bring to the boil. Reduce the heat and simmer for 12-15 minutes until a knife goes through the knife easily. 

Meanwhile heat the oil and 25g of the butter in a pan over a medium heat and cook the sliced sprouts and spring onions for a couple of minutes until soft.  Put to one side.

In the same pan, add 75g of the butter and gently fry the shallot and leek for 5 minutes and then add the flour.  Cook for a further minute and then gradually add the milk, stirring, until you have a smooth sauce. Season and bubble for 5 minutes until thick and glossy. 

Drain the potatoes when cooked and put back into the pan.  Put the pan on a low heat and mash until very smooth.  Then when you think it is smooth enough, mash again.  The extra mashing really will remove the lumps - once the butter and cream are in the potatoes you will not get any lumps out although I really don't know why that is.  Add 25g of butter and stir until the butter is melted.  Add 75-100ml of the cream, the cheese and the vegetables and stir.  Check the seasoning and adjust if needed.

Add the remaining cream, the parsley and the nutmeg to the béchamel, and stir.  Stir the ham through the sauce and check the seasoning. 

Put the ham and parsley sauce into an ovenproof dish and top with the bubble and squeak.  Put into the oven for 30 minutes until the mash is golden brown and the filling bubbling. You should put the oven dish onto a baking tray so it does not end up all over the bottom of your oven.  I say this through experience.