Heat the oven to 180C/160C fan/gas 4. Oil the base of a 950g loaf tin and line with baking parchment. Heat the oil in a frying pan over a medium heat and fry the onion and mushrooms for 8-10 mins until the mushrooms have released their liquid and browned. Tip into a large bowl and leave to cool for 5 mins before adding all the remaining ingredients, except the cranberry sauce. Season well and mix to combine.

Spoon the mixture into the tin and bake for 45-50 mins until cooked through and golden on top. If the top begins to brown too much before it's cooked through, cover with foil. Turn out of the tin onto a serving board or plate, and remove the parchment. Leave to cool for 5 mins, then top with the cranberry sauce (if using), slice and serve warm or cool completely and eat cold.