This soup extols the essence of watercress; fresh, peppery, and nutritious.
We add ice to the watercress to stop the cooking process to retain the vivid colour and the freshness of the flavour.
The watercress will have a different degree of pepperiness, so taste before cooking, accordingly, leave more stalks if liked but cut off all the stalks if strong.

Serves (Yield):   4 Difficulty rating: ●○○
Preparation time:   5 mins Cooking time: 15 mins
Special equipment: 
Planning ahead: This soup can be prepared one day in advance and reheated to order. 

Ingredients:
1/4     White Onion, finely chopped
20g    Unsalted butter
2  Large bunches watercress, stalks removed*1
100g  Spinach, picked and washed
400g   Boiling water *2
6g   Sea salt
Ground black pepper, freshly ground
400g    ice or iced water

Method:
In a large chef’s pan with a lid, on a low heat, sweeten the onion for 3 minutes in the butter until soft and translucent.*3 Turn up the heat, add the watercress and spinach, cover and wilt for 2 minutes.  Add the boiling water, season with salt and pepper and bring to a quick boil for 2 minutes. Take off the heat, add the ice to stop the cooking and purée in a liquidiser until very smooth. *4
Reheat, taste and correct the seasoning if required – serve immediately, piping hot to your guests in a large soup tureen.

RB's notes (*):
*1  According to the variety, the watercress may vary in its strength of flavour.  If your watercress is very peppery, cut off all of the stalk, but if it is mild, keep some of the stalk.
*2  The boiling water speeds up the cooking of the soup and retains the colour and flavour of the watercress.
*3 Sweating the vegetables will release the flavour and make it easier to digest.
*4 The ice stops the cooking, which will again keep the vivid colour, flavour and the nutrients. Reheat the soup at the last moment for the same reasons.
Variations:
As a good Frenchman, I would be tempted to add a clove of pureed garlic or flakes of toasted garlic to the soup. A tablespoon of Greek yoghurt would be a nice addition. A squeeze of lemon juice added at the last moment would heighten the flavour of the watercress, but add at the last minute or it will discolour the watercress.
Nutritional info:
Onions – are a complex carbohydrate also known as polysaccharides; a soluble fibre for the good bacteria in the stomach and reduces bad cholesterol.
Spinach & Watercress – are a good source of Folic acid, and being a Dark Green Leafy Vegetable (DGLV) it is full of antioxidants, Vitamin A, good for your eyes, skin. And full of Iron, which prevents anaemia & fatigue.