2 Spring garlic heads (with stalks attached if possible)
110g Unsalted butter
1 Large Spanish onion (about 450g, halved lengthwise and thinly sliced)
2.5 tablespoons Maldon salt (or another flaky sea salt)
450g Baking potatoes (peeled, cut into 1-cm pieces, rinsed and drained well)
450ml Whole milk
450g Watercress (bottom 2.5cm of stems trimmed, thick stem thinly sliced, the rest left whole)
1 Small handful of chervil springs (optional)
110ml Extra virgin olive oil
120g Crème fraîche
1/2 tablespoon Black peppercorns (toasted in a pan until aromatic)
Trim away the roots and cut off and discard the dark green tops of the garlic. Halve the bulbs lengthwise and thinly slice them.
Melt the butter in a medium pot over medium-low heat, then add the garlic, onion, and 1 tablespoon of the salt, and have a stir. Cover the pot and cook, stirring occasionally, until the onion is very soft and creamy (about 30 minutes).
Add the potatoes, milk, the remaining 1.5 tablespoons of salt, and 450ml of water. Cover the pot, increase the heat to medium, and bring the liquid to a vigorous simmer. Uncover and lower the heat if necessary to maintain a gentle simmer. Cook until the potatoes are fully tender, about 15 minutes.
Add the watercress (and chervil, if using) and gently stir to submerge the greens in the liquid. Bring the mixture back to a simmer and add the 110 ml oil, then remove the pot from the heat.
Pour the soup into a large mixing bowl and blend the soup until very smooth (working in batches if necessary and adding each batch back to the pot).
Combine the crème fraîche and crushed peppercorns in a small bowl with 1 teaspoon of olive oil and a pinch of salt and stir well. Serve with the crème fraîche mixture on top.