Preheat the oven to 180°c/350 Fahrenheit/gas 4
To make the meat sauce, first heat the oil in a large pan over a medium heat and fry the onion until it starts to soften, then add in the garlic and stir.
Add the mince and brown the meat, breaking it up with the back of a wooden spoon or spatula as it changes colour.
Add the carrots and celery and cook for 5 minutes, stirring well.
Pour over the beef stock and mix well.
Season well and add the bay leaves, then allow to simmer for about 25 minutes.
Add in the mushrooms, if using, and a few splashes of Worcestershire Sauce, mix well then simmer uncovered for another 20 minutes
Meanwhile, make the Clapshot - peel and chop your potatoes and turnip/swede into even sized chunks. Place in a pot with a dash of salt, cover with water and bring to the boil. Boil for about 20 minutes, until you can easily slice a piece in half with a fork.
Drain the vegetables and place in a large bowl, then cover with a clean tea towel and set aside for 5 minutes.
Add the butter in small chunks and a generous grind of black pepper, then mash everything together. Add salt to taste.
When the meat sauce is ready and has thickened nicely, remove the bay leaves and pour it into the casserole dish in a smooth, even layer.
Spoon the mash over the sauce and smooth out so it covers the whole dish.
Bake in the preheated oven for about 30 minutes, until the top is starting to turn golden.
Sprinkle over some fresh chives or parsley and serve, ideally with some buttery peas or steamed broccoli.