Step 1

Preheat oven to 180°C. Grease a baking tray and line with baking paper.

Step 2

Wrap each beetroot in foil and place on prepared tray. Roast for 2-2 1/2 hours or until a knife pierces them easily. Remove and cool, then peel (wear gloves to prevent staining) and roughly chop.

Step 3

Meanwhile, to make the stock, heat half the oil in a saucepan over medium heat. Add coriander seeds and cook, stirring, for 1-2 minutes or until fragrant. Add half the onion, leek and garlic, and cook, stirring occasionally, for 3-4 minutes or until slightly softened. Add the celery, carrot and fennel, and cook, stirring occasionally, for a further 2-3 minutes or until carrot begins to soften.

Step 4

Add vinegar, half the thyme and 2.5L (10 cups) water. Bring to a simmer, then reduce heat to low and cook, skimming impurities from the surface, for 45-60 minutes or until fragrant and flavours are infused. Remove from the heat and allow to cool slightly, then carefully strain through a fine sieve, discarding vegetables. Set aside.

Step 5

Heat remaining 1 1/2 tbs oil in a large saucepan over medium heat. Add the remaining onion, leek and garlic, and cook, stirring, for 2-3 minutes or until translucent. Add beetroot, potato and remaining thyme, and cook, stirring, for a further 2-3 minutes or until warmed through. Add stock, then increase heat to high and bring to a simmer.

Step 6

Reduce heat to medium and cook, stirring occasionally, for 15 minutes or until potato is tender. Cool slightly, then, in batches, whiz in a blender until smooth (add a little hot water if needed).

Step 7

Serve soup topped with feta, olive oil, coriander seeds and lemon thyme.