Finely chop the garlic. Clean, trim and roughly chop the leeks, broccoli & carrots. Peel the celeriac and roughly chop.
Heat the oil in the cooker over a medium heat then add the garlic. Stir to soften then lower the heat, add the leeks and sauté for a couple of minutes.
Put in the remaining vegetables and allow to sweat for 5 minutes, stirring occasionally.
Add the stock, bring to the boil then cook under pressure for 8-10 minutes. Cool under running water.
Use a hand held blender to mix to a smooth consistency. Simmer and stir in the open pan for another 5 minutes adding a little more stock if you prefer a thinner soup.
Scatter some fresh herbs over each portion.