Crème de cresson

03 — Method

1. Wash the 2 large bunches of watercress carefully in several changes of cold water to remove any soil hiding between the leaves. This is ABSOLUTELY essential: watercress holds a lot of sand and tiny bits of grit.
2. Pick the watercress, keeping the tender green leaves and thin tender stems. Discard the thick, fibrous stems. Set aside ¼ of the watercress to garnish at the end as a fresh chiffonade.
3. Peel the 2 yellow onions and slice them thinly into even half-moons. Remove the green germ from the 3 garlic cloves and chop them with a paring knife. Peel the 400 g (14 oz) of potatoes and cut them into even 2 cm (¾ in) cubes so they cook evenly.
4. Melt the unsalted butter with 3 tbsp of olive oil in the large saucepan over medium heat without browning. Sweat the sliced onions for 8 minutes until translucent. Add the chopped garlic and cook for 1 minute, stirring, to release its aroma.
5. Add the potato cubes and cook for 3 minutes, stirring, so they soak up the flavors. Pour in hot chicken stock to just cover the vegetables. Season with fleur de sel and plenty of pepper halfway through cooking.
6. Bring to a simmer and cook gently for 20 minutes until the potatoes are very tender. Add ¾ of the watercress and cook for just 3 minutes so it keeps its bright green color.
7. Blend the soup with the immersion blender right in the pan until perfectly smooth, velvety and spring green. Press it through a fine-mesh strainer with a spatula to remove any fibers — that silkiness is what makes this cream soup special.
8. Off the heat, whisk the egg yolk with 100 ml (⅓ cup) of thick crème fraîche to enrich and thicken the soup. Gradually whisk it into the warm soup. Serve sprinkled with the watercress chiffonade, chives and garlic croutons.