Pre-heat oven to moderate (180 degrees celsius/355 degrees fahrenheit).
Fry onion in oil over low heat until translucent.
Add beef mince and increase heat to med-high. Cook until browned, chopping mince as it cooks.
Add carrot and celery and cook for 1-2 minutes.
Add beef stock, tomato paste and Worcestershire sauce and bring to a simmer. Simmer for 5-10 minutes. Add a teaspoon of flour mixed into a tablespoon of water towards the end of the simmering time to thicken the sauce if necessary.
Peel and chop potatoes into cubes. Boil (adding the stock cube to the water) until tender.
Drain potatoes and then add the butter, egg and ⅔ of the cheese while mashing the potatoes.
Pour mince mixture into the casserole dish. Then spoon potato mash over the top, smoothing to ensure mince mixture is completely covered. Sprinkle remaining cheese on top.
Cook in oven for 40 minutes.
Notes
Storing:Cottage Pie can be enjoyed hot or cold and can be kept for up to a few days in the fridge and re-heated.Freezing:You can freeze leftover Cottage Pie for up to three months. Wrap well and then freeze.Nutrition:The nutrition information is an automatically generated estimate and is not guaranteed to be accurate.Tips:Add a teaspoon of flour mixed into a tablespoon of water towards the end of the simmering time to thicken the sauce if necessary.