Peel the shallots, place in a bowl and pour over boiling water. Leave for one minute, then drain. This will make them easier to peel. Top and tail the shallots and thickly slice.
Heat the butter in a pan and add the shallots when foaming. Cook the shallots very gently until soft and translucent – about 15-20 minutes.
Add the sugar, garlic and thyme and stir to coat and cook for a further five minutes so the shallots caramelise. Pour over the stock and bring to the boil. Simmer gently until the stock is reduced to a syrupy consistency and the shallots are very tender.
Remove the thyme sprigs and season before serving.