Peel and finely chop the onion. Trim the roots and top 3cm off the leek. Halve it and rinse out any grit. Finely slice it. Trim the dry ends off the celery sticks and finely slice them.
Put a large pan over a medium-low heat. Warm for 1 min, then add 2 tbsp oil and the chopped veg. Season with a pinch of salt and pepper. Cook for 10 mins, stirring occasionally, till the veg started to soften and brown.
While the veg sweat, trim the carrots, peel and thickly slice them on the diagonal. Trim and peel the parsnips. Chop them into 2cm thick chunks. Stir the carrots and parsnips into the veg with 3 tbsp water and fry for another 8 mins, stirring occasionally.
While the veg fry, peel and chop the potatoes to match the parsnips. Peel and crush the garlic. Crumble the stock cube into a jug and stir in 1 ltr boiling water.
Stir the potatoes and garlic into the veg. Pour in the stock. Tip in the tin of tomatoes and add the tamari. Swirl in the pearl barley and drop in the bouquet garni. Pop on a lid (use a baking tray to cover the pan if you don't have a lid), turn up the heat and bring to the boil.
When the hot pot is boiling, turn the heat down so it's gently simmering. Simmer for 30 mins till the potatoes and pearl barley are tender. Taste and adjust the seasoning. Fish out the bouquet garni and ladle the hot pot into warm bowls to serve.