Making soup is often a good strategy for using up the forgotten vegetables that are going limp and yellow at the back of the fridge. I’ve never really warmed to the generic “vegetable soups” that contain a whole host of different roots and greens, all puréed together into a (usually) brown slop tasting just of overcooked veg. Much better are soups made from one or two dominant vegetables, perhaps with a choice herb or spice added to the mix, like carrot and coriander, or leek and potato. Onion is always the base, perhaps with a little celery or carrot to enhance the stock, then with a little potato or red lentils (usually depending on the colour of the main ingredient) to add body.
I’ve never really had much luck with making soup with leafier vegetables though. I always think of soup as a slow process and end up overcooking the greens and ending up with a sad and slightly sour meal that fails to satisfy. Making this broccoli soup however, I decided to cook the florets as little as possible so that they not only retain a bright green colour, but also some of that freshness and pep they have when raw. The pep itself comes from the same family of chemicals that give heat to mustard, horseradish and fiery wasabi which, perhaps somewhat ironically, evolved as a deterrent to the chomping of herbivorous animals: isothiocyanates.
Isothiocyanates tend to degrade quite quickly when exposed the air (the vinegar used in mustard is to prevent this degradation) or when heated, hence why broccoli, cabbage and other brassicas aren’t spicy once it has been cooked. This soup, made more like a nettle or watercress soup by blanching the greens, is my attempt at trying to preserve the piquancy of raw broccoli. You could of course adapt it to any kind of leafy green though.
In a large saucepan, warm the cooking oil and sweat the onion and celery until softened but not coloured, about 10 minutes. Add the potato and stock and bring to boil. Simmer until the potatoes are tender.
Meanwhile, bring a large pan of water to the boil. Add the broccoli to pan, stalks first, and cook until bright green and just beginning to become tender: this keeps the finished soup a bright green, and the blender removes the need to cook the broccoli until soft. Drain, and rinse the broccoli under cold water (or submerge in an ice bath) to stop the cooking.
Once the potato is cooked, remove the soup base from the heat. Leave to cool slightly before adding the broccoli and blitzing to a smooth soup with a stick blender (if using an upright, lidded blender or food processor let the soup cool more before puréeing to limit the steam build-up under the sealed lid). Adjust the thickness of the soup by adding more stock (or cream to make it richer), and taste and season with salt and lots of pepper. Before serving, bring back up to a simmer so it is piping hot. Serves 4.