Put 400g/14oz onions, 400g/14oz wild garlic and the thyme in a large saucepan with 800ml/1½ pints water and bring to the boil. Simmer for 30 minutes, remove from the heat, cover and leave to infuse for 2 hours.
Preheat the oven to 180C/160C Fan/Gas 4. Wrap the garlic bulbs in kitchen foil and roast for 30–40 minutes, or until softened. When cool enough to handle, squeeze the garlic flesh into a bowl and set aside.
For the bacon-fat potatoes, heat 2 teaspoons olive oil in a frying pan over a medium heat. Fry the pancetta, potato cubes, thyme and crushed garlic for 3–6 minutes, or until golden-brown and crispy. Remove from the heat, drain on kitchen paper and set aside.
For the crispy skin, melt the butter in a small saucepan and pour over the potato skin. Season with salt. Fill a small heavy-bottomed saucepan one-third full with oil. Heat to 180C (a cube of bread should turn brown in 1 minute when dropped in.) CAUTION: Hot oil can be dangerous. Do not leave unattended. Deep-fry the potato skin until crispy, then remove with a slotted spoon and leave to drain on kitchen paper.
For the soup, melt the butter in a large saucepan over a low heat and cook the remaining 100g/3½oz onions for 5 minutes, or until softened. Add the potato slices and roasted garlic and cook without colouring until the potato is soft.
In a separate saucepan, heat the double cream with the wild garlic stock, then pour onto the onion and sliced potato mixture. Add the remaining wild garlic stalks and bring to the boil. Blend the soup in a blender, strain through a sieve back into the pan and season with salt.
To serve, place a spoonful of sour cream into the middle of each of four shallow bowls, top with wild garlic oil, if using, then the bacon-fat potatoes, crispy skins and wild garlic leaves and drizzle the soup around the edges of the bowls.