Heat the butter in a large, heavy-based saucepan set over a medium heat. Sweat the onion with some salt for 6-7 minutes, stirring occasionally, until softened. Add the roughly chopped garlic cloves and the thyme and sweat for 3-4 minutes, stirring regularly making sure the garlic doesn't colour.
Add 1.4ltrs of boiling water to the saucepan and bring to the boil. Reduce to a simmer for 15 minutes, then add the crème fraîche and whisk well. Remove the soup from the heat and discard the thyme sprig. Use a stick blender to purée the soup until smooth.
Return to the heat and adjust to taste using the white wine vinegar and salt. Keep the heat over a low heat as you prepare the garlic garnish; heat the olive oil in a frying pan over a moderate heat. Fry the sliced garlic until golden brown, then remove to a plate lined with kitchen paper to drain.
Ladle the soup into soup bowls and garnish with the fried garlic and some black pepper before serving.