Heat the oven to 180°C Cut the sweet potatoes in half lengthwise, rub with the olive oil and bake, cut side down for 1 hour until tender. Allow to cool, then scoop the flesh out of the skins and discard the skins.
Arrange the bacon on a foil-covered baking tray and grill or fry in a non-stick frying pan until super crisp. Remove and drain on kitchen paper, then crumble into small pieces. Cook the onions in the bacon fat or in 1 tbs olive oil over a moderate heat for 5 minutes until they start to soften.
Add the sweet potato flesh, bay leaves, chicken stock, nutmeg, salt and pepper and bring to the boil, stirring. Simmer for 15 minutes, stirring occasionally. If it’s too lumpy for your taste, whip out the bay leaves and whiz the whole lot in the food processor, otherwise leave it alone. Serve in warmed soup bowls, topped with crumbled crisp bacon.