Radish leaves give this light soup a fresh, peppery flavour. Lemon brightens it up, while green herb oil and crisp toasted bread add extra aroma and texture.
Radish leaves are tender and mildly peppery, and are usually at their best when fresh and firm. Bunched radishes often still have their leaves attached; in the Netherlands, radishes are most readily available from spring through autumn. Ideally, use the leaves on the day you buy them or shortly afterwards. Remove them from the radishes, rinse them thoroughly in a large bowl of cold water, and check for grit between the stems. It’s best to discard any limp or yellowing leaves. This recipe uses the leaves as the main ingredient, not just as a garnish: 120 grams may seem like a lot, but they wilt quickly as soon as they come into contact with hot stock.
The flavour is green and mildly peppery, not sharp like a generous amount of raw radish can be. Gently softening the onion in oil gives the soup a rounded base without overpowering its fresh flavour. The leaves only need to wilt before you blend them. Cooking them for too long dulls the colour and can flatten the fresh flavour, so take the pan off the heat as soon as the leaves are soft. An immersion blender works right in the pan, but blend carefully and keep the head below the surface to limit splashing. Add the lemon juice a little at a time: stock varies in saltiness, and lemons vary in juiciness.
The herb oil brings a different kind of green to the soup: aromatic and fresh, with a smooth, oily texture. Flat-leaf parsley is reliable and easy to find; you can replace some of it with chives for a gentler, onion-like note. Use a mild olive oil for the herb oil so it doesn’t overpower the herbs. The soup already has lemon, so the oil doesn’t need any extra acidity. Make only as much as you need, and don’t leave any leftovers at room temperature for long; cover and refrigerate them, and use within a day.
Thin strips of baguette toast quickly and provide a crisp contrast to the soft soup. Cut them evenly so the tips don’t burn before the wider pieces turn crisp. Sourdough bread works too, as long as you cut it into narrow strips. For a gluten-free version, leave out the bread or serve it with a gluten-free alternative. The soup is intended as a starter, but with extra bread and a simple salad, it can also make a light lunch. Cool leftover soup promptly, covered, and keep it in the refrigerator for up to two days; reheat until piping hot throughout.
Preheat the oven to 200 °C. Cut the baguette into thin strips, toss them with 1 tablespoon olive oil and spread them out on a baking sheet. Toast for 8–10 minutes until golden brown and crisp, turning halfway through.
Heat 1 tablespoon olive oil in a soup pot over medium-low heat. Cook the onion for 5–6 minutes until soft and translucent, without letting it brown.
Pour the vegetable stock into the pot and bring to a boil. Remove the pot from the heat, add the radish leaves in batches and stir until just wilted.
Blend the soup until smooth with an immersion blender. Stir in the lemon zest and juice, and season with black pepper. Taste before adding extra salt; stock is often already salty.
Meanwhile, blend the parsley with the mild olive oil and fine salt until you have a green herb oil. Divide the soup between four bowls, drizzle the herb oil over the top and serve with the toasted bread strips.
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