Set a large saucepan with the oil over a medium heat. Fry the onion for 5 minutes until soft and translucent. Stir in the mushrooms and cook for 2 minutes more. Add the beef and thyme and cook until the meat has browned all over.
Pour in the stock and add the bay leaf. Bring to a gentle simmer, cover and cook for 1 hour 30 minutes, stirring occasionally, until the meat is really tender and you have a thick gravy.
Preheat the oven to 180°C, gas mark 4. Spoon the filling into a small ovenproof dish.
Put the butter and cream or milk in a saucepan set over a medium heat. Once the butter has melted, stir in the roasted vegetables and chopped garlic and mash roughly.
Top the pie filling with the mash and cook for 20-25 minutes, until the filling is bubbling and the topping is golden.