Method

1. Heat the oil in a sauté pan or large frying pan, add the onion and carrots and cook over medium heat for about 5 mins, stirring occasionally, until the vegetables have started to soften. 
2. Add the beef to the pan and cook for about 5-10 mins, stirring to break up the meat, until it browns. Once the meat is crumbly, stir occasionally to prevent it from sticking and burning, but don’t keep stirring it, otherwise, it just cools the mixture, rather than letting the meat brown. 
3. Add the mushrooms to the pan and cook for a few mins.
4. Sprinkle over the flour, then mix it in well and heat for 3-4 mins to cook the flour. The flour will absorb the fat that comes out of the meat, which will then thicken the sauce.
5. Pour in the can of tomatoes, sprinkle in the stock cube, and bring the mixture to a boil, stirring continually, then reduce the heat and simmer for about 10 mins, until it thickens slightly and the vegetables are tender. If the sauce seems very thick, just add a little boiling water.
6. Set oven to 200°C/400°F/Gas Mark 6.
7. Stir the Worcestershire sauce into mince, then spoon into a dish and level the surface.
8. Beat the potato to soften it and spread over the mince. Score the surface with a fork and put butter on top.
9. Bake in the center of the oven for about 20-30 mins, or until the pie is golden and the filling is bubbling. For a crisp, golden finish, sprinkle your cottage pie with lashings of cheddar cheese and pop it under the grill for a few minutes longer. Serve straight from the oven.