Shepherd's Pie

Shepherd's pie is usually consuming during winter season in Australia. However, since I got plenty of celery, carrot and potato in my fridge and I run out the ideas of meal.
This meal suddenly come up when I read a copy of recipe in the Local Community centre. The lady was so nice to allow me to make a copy of the recipe. However, I just adjusted to what such of ingredients that are available at home. And, here is the my home made Shepherd's pie.

Ingredients:
300 gr mince beef
1 carrot, finely chopped
2 stalk celery, finely chopped
2 tbs butter
2 brown onions, finely chopped
1/4 cup AP Flour
1/2 tsp Dijon Mustard
1 1/2 cup beef stock
2 tbs Worcestershire sauce
1 tsp sugar
salt and black pepper

potato topping:
1/2 cup hot milk
25 gr butter
salt and black pepper to taste
5 large potato, boiled and mashed
some mozarella cheese

Directions:
Preheat the oven to 210'c. Melt the butter in a large pan, add the onion and cook until soft. Brown the mince meat, add some carrot and celery. Sprinkle the flour, mustard and Worcestershire sauce. Gradually add the stock and stir constantly. Seasoned with sugar, salt and pepper. Bring it to boil and simmering for 5 minutes until the veggies are soft. Remove from heat and spoon into the casserole dish.

To make a potato topping:
Just mix all the ingredients, except the mozarella cheese. Spread the mixture over the meat. Sprinkle with cheese. Bake for 45 minutes.