Peel and finely dice the shallot. Separate the watercress leaves from the stalks and place in two bowls. Peel and chop the garlic. Peel and dice the potatoes. Peel and dice the celeriac.
Heat a saucepan and melt the butter. Add the shallot and sweat with a pinch of salt. Once transparent, add the garlic, the potatoes, the celeriac and the watercress stalks. Cook for a few minutes and then deglaze with the white wine. Reduce the wine by half. Add the vegetable stock, bring to the boil and cook until the potatoes and celeriac are soft. Blitz in a food processor until smooth and then add the watercress and spinach leaves and continue blitzing until bright green. Check the seasoning.
2For the crostini
Preheat the oven to 180'C.
Thinly slice the spring onions. Cut the French stick into thin slices. Drizzle with olive oil and then bake in the oven for 10 minutes until golden brown and crispy.
Grate the cheddar. Mix the spring onions with the cheese and add a splash of Worcestershire sauce. Sprinkle the cheese mixture on to the bread and then melt the cheese under a hot grill.
Serve the soup with a crostini floating in the middle.
« To make sure the soup is a vibrant green colour, blitz the watercress leaves into the soup at the very last minute. »