Bring a large pain of water to the boil. Add the salt, following by the chunks of potato and sweet potato. Reduce heat so that it is a gentle boil, and cook for 5 minutes.

Then, add the parsnip and boil for a further 5 minutes. Finally, add the carrots and simmer for about 10 minutes, or until the potato starts to come apart when prodded with a fork. The texture needs to be firm, but not hard, and certainly not boiled to a pulp. The carrots will be firmer - this is intentional.

Carefully strain the vegetables in a colander, then rinse off the starchy scum with water.

Next, wipe down the inside of the saucepan and heat up the olive oil. add the onions, and sweat on a medium heat for 2- 3 minutes. Then, add the grated garlic and continue to cook until soft. Remove from heat.

Place the vegetables in a large mixing bowl and gently mash with a large fork or slotted spoon. Everything needs to be combined evenly, but not mashed to a paste. Then, add the onions and mix into the root vegetables. Dab small amounts of the wholegrain mustard over the mixture, and fold in evenly. This will help with the even distribution of the mustard, rather than having a huge dollop in one helping. Add the chopped parsley, season to taste, and combine one more time.

Line a baking tray with greaseproof paper, and grease lightly with a little olive oil. Put a gently greased chef's ring on top of the greaseproof paper, and carefully spoon the mash into the centre. Compress the mixture until the mash is compacted, then using a plunger hold down and twist off the ring. If you keep rotating the plunger gently, it is less likely that the bottom of the plunger will stick to the top of the mash. Repeat, until the mash is used up. It should make 6 generous helpings.

Finally, place in the top of an oven (around 160C) and cook for around 30 minutes until piping hot.