Rabarbersuppe

Ingredients

Equipment Needed

• •Heavy-bottomed saucepan, 3 litre
• •Small bowl for the potato starch slurry
• •Balloon whisk
• •Shallow serving bowls or tall glasses

Instructions

1. 1

   Prepare the rhubarb

   Trim the rhubarb by cutting off the leaves (they're mildly toxic and should never be eaten) and the dry ends of the stalks. If the outer skin looks tough or stringy, peel it lightly with a small knife. Young early-season rhubarb rarely needs peeling; its skin is thin and tender and carries most of the pink. Cut the stalks into pieces about two centimetres long. Uniform pieces cook at the same pace, and that's what you want.

   If you can, choose the thinnest, reddest stalks at the market. They're younger, sweeter, and give the soup its deep pink. Thick pale stalks later in the season work too, but you'll need more sugar and a slightly longer cook.
2. 2

   Simmer with vanilla

   Put the water, sugar, and split vanilla pod into a heavy saucepan. Scrape the vanilla seeds into the liquid with the tip of a knife, then drop the pod in too. Bring everything to a gentle simmer and stir until the sugar dissolves completely. Add the rhubarb and let it simmer gently for eight to ten minutes. You want the pieces tender but still holding their shape. Rhubarb turns to mush quickly if you look away, so check it early. When a piece yields easily to the back of a spoon but doesn't fall apart, it's ready.

3. 3

   Mix the potato starch slurry

   While the rhubarb simmers, stir the potato starch together with three tablespoons of cold water in a small bowl until completely smooth. This is called an opslaemning, the Danish word for a cold slurry. The starch needs cold water to dissolve evenly; if you stir it into the hot soup without this step, you'll get lumps that never smooth out.

   Kartoffelmel is the traditional Danish thickener for sweet soups and sauces. It gives a glossier finish than cornstarch and has no aftertaste. Most supermarkets carry it; Scandinavian shops always do.
4. 4

   Thicken off the heat

   Take the pan off the heat completely. This is the most important moment in the whole recipe, so pay attention. Stir the slurry once more to re-suspend the starch, then pour it into the hot soup in a slow steady stream, whisking gently as you go. Return the pan to a low heat and warm the soup through for about a minute, stirring constantly. You'll see it change: the cloudy liquid turns glossy and silken, and it coats the back of a spoon. Stop there. Don't let it boil. Boiling breaks the potato starch and the soup loses its body and goes thin again.

   You'll know when it's right: the soup holds a soft sheen, and when you lift the spoon, the drip is slow and rounded, not watery.
5. 5

   Chill completely

   Pour the soup into a wide bowl and let it cool to room temperature. Fish out the vanilla pod, rinse it, and save it for another use (dropped into a jar of sugar, it perfumes everything it touches for months). To stop a skin forming on the surface as it cools, either press cling film directly against it or sprinkle a fine dusting of sugar over the top. Chill in the fridge for at least three hours, preferably longer. This soup is meant to be cold, truly cold. Lukewarm rabarbersuppe is not the dish you're after.

6. 6

   Whip the cream softly

   Just before serving, whip the cold cream with the icing sugar, if using, until it holds soft peaks. Soft peaks, not stiff. You want the cream to fall from the spoon in lazy folds, not stand up like meringue. The cream is going to sit on the surface of the soup and slowly melt into it, and soft peaks do this far better than stiff ones. Danish whipped cream is almost always underwhipped on purpose, and you'll understand why the first time you eat it.

7. 7

   Serve and linger

   Ladle the cold soup into shallow bowls or tall glasses, making sure each portion gets some of the rhubarb pieces suspended in the pink. Spoon a generous cloud of whipped cream into the centre of each one. If you have kammerjunkere, the small twice-baked Danish biscuits made for exactly this purpose, scatter a few over the top or serve them in a bowl alongside for dipping. Toasted flaked almonds work beautifully too. Eat slowly, outside if the evening allows, while the long northern light holds. Tak for mad.

Chef Tips

• •Taste your rhubarb raw before you decide on the sugar. Early-season stalks are tart but clean; late-season ones need more sweetening and sometimes a squeeze of lemon to brighten them. The season decides, and you decide with it.
• •If you want a deeper pink, set aside a handful of the reddest rhubarb pieces and cook them separately for just three minutes, then stir them in at the end. They hold their color and give the soup a visual lift.
• •Kammerjunkere, the small twice-baked biscuits traditionally served with cold fruit soups, are worth seeking out at a Danish bakery or making from scratch. They soften slightly when dropped into the soup, and that contrast of crisp and cold and creamy is the whole joy of the dish.
• •Rabarbersuppe is also a very good breakfast the next morning, cold from the fridge with a spoonful of thick yogurt instead of cream. Nobody will tell on you.

Advance Preparation

• •The soup can be made up to two days ahead and kept covered in the fridge. The flavor deepens as it rests, and the color holds beautifully.
• •Whip the cream fresh, just before serving. Pre-whipped cream weeps and loses its soft folds, and that changes the whole feel of the dish.
• •Kammerjunkere keep for a week in an airtight tin, so bake them in advance if you're making this for a dinner party.

Frequently Asked Questions

Nutrition Information

1 serving (about 420g)