Heat the oil and butter in a largeish non-stick frying pan over low–medium heat until the butter has melted. Add the onion and cook for a good 5–10 minutes until is really softened.
Add the potatoes and parsnips and use a potato masher to mash them in the pan to a rough, chunky mash (turn the heat off while you do the mashing).
Add the turkey, bacon, carrots and Brussels sprouts to the pan, along with some salt and pepper (how much will depend on how much you seasoned everything the day before), and mix everything together.
Press the mixture down gently until it’s compacted into one large patty. Cook for about 6 minutes, until the bottom of the patty has turned a golden brown.
Carefully, and using oven gloves, place a plate on top of the pan and invert the patty onto the plate.
Slide it back into the pan and cook for a further 6 minutes or so, until the other side is golden brown and it’s piping hot throughout. Serve hot with a dollop of cranberry sauce.