Pat the ox cheek dry with a clean tea towel or kitchen paper, then season well. Heat 1 tbsp of the oil in a large pan over a medium heat and brown the ox cheek all over, around 6-8 mins. Remove from the pan, leaving any oil behind.
Drizzle the remaining oil into the pan and turn the heat down to medium-low. Tip the carrots, onion and celery into the pan, stir and cook for 10-12 mins until softened but not golden. Stir in the garlic, cook for 1 min, then tip in the flour and tomato purée and cook for 1 min. Pour in the wine, cook for a few minutes to evaporate the alcohol, then add the stock, Worcestershire sauce, thyme and bay leaf. Put the ox cheek back into the pan, then cover and simmer for 3 hrs until tender. (You could also transfer everything to a slow cooker at this point and cook for 5 hrs on high or 7 hrs on low.)
About 30 mins before the ox cheek has finished cooking, bring a large pan of salted water to the boil and cook the potatoes for 15-20 mins until tender. Drain and leave in the colander for around 10 mins to steam-dry a little. If you’re not freezing this dish ahead, heat the oven to 180C/160C fan/gas 4. Mash the potatoes well, or press through a ricer to ensure they are finely mashed. With the mashed potato back in the pan, pour in the milk, add the butter, grate in a little nutmeg and stir until the butter has melted in the residual heat.
Shred the ox cheek in its gravy using two forks so that it’s broken down a little – this will help ensure the potato sits evenly on top. Tip the ox cheek mixture into a freezable ovenproof dish, then spoon over the mash. Use a fork to drag ridges through the mash, then scatter over the cheese. Bake for for 30-40 mins until the topping is golden. To freeze the pie, leave to cool completely after adding the mash topping, then cover with foil. Will keep frozen for up to three months. Cook from frozen at 180C/160C fan/gas 4 for 1 hr–1 hr 15 mins until piping hot throughout. Serve with cooked greens, if you like.