To make the sauce place the onion, carrot and celery in a large saucepan with around half a cup of water.
Put the lid on the pan and cook over a low heat (topping the water up if necessary) until the vegetables are soft
Add the lamb mince and garlic and fry over a medium heat until well browned
Finally add the remaining sauce ingredients, turn the heat down low and allow to simmer with the lid on for around 30 minutes
To make the topping steam the cauliflower florets for around 10 - 15 minutes or until really soft
Meanwhile fry the spring onions and garlic in a little oil over a low heat for a couple of minutes until softened, but not coloured
Once cooked add the cauliflower florets to the spring onion and garlic mixture and fry for a further couple of minutes over a low heat
Finally, place the cauliflower, onion and garlic mix in a blender with the mustard salt and pepper and blend until smooth and creamy. You may need to add a tiny bit of water...but don't add too much
Assemble the pie by dolloping the cauliflower on top of the sauce mix.
If you want to top with cheese you can do...I suggest placing the pie under a grill in this case to melt and brown the cheese