Quarter the celeriac. Slice off the skin and knobbly bits. Cut into 2-3cm chunks. Halve, peel and finely chop the onion. Peel and finely chop or grate the garlic.
Set a large pan over a high heat to warm for a few mins. Add 1 tbsp oil to the pan, followed by the veg and garlic. Season well with salt and pepper. Sizzle and stir for 5 mins.
Pour in 1 ltr boiling water. Add the stock cube. Simmer for 20 mins or till the celeriac is tender.
While the soup simmers, peel, core and roughly chop the pears. Roughly chop the parsley. Zest the lemon.
Ladle the soup into a blender and add the pears, parsley, lemon zest and a squeeze of lemon juice. Blitz till smooth. Alternatively, add everything to the pan and use a hand-held blender to blitz. Taste and add more seasoning or lemon juice if you think it needs it. Serve in warm bowls.