Peel and slice 1 garlic clove. Peel and roughly chop the sweet potatoes. Heat a large pan for 1 min over a low heat. Add 1 tbsp oil. Add the garlic and a pinch of the rosemary leaves. Add a pinch of salt and fry for 2-3 mins till the garlic is golden and crisp. Lift out of the pan with a slotted spoon and place in a bowl.
Add the sweet potatoes to the pan and cook for 5 mins, stirring often, till caramelised and browned all over.
While the potatoes cook, peel and finely chop the onion and remaining garlic cloves. Add to the pan and cook for 5 mins till slightly softened. If the veg stick, add a splash of water. Stir in the remaining uncooked rosemary leaves. Crumble in the stock cube. Pour in 800ml boiling water. Pop on a lid and simmer for 20 mins.
Ladle the soup into a blender and blitz till smooth, or use a hand-held blender. Taste and season with salt and pepper. Ladle into warm bowls. Scatter over the crispy garlic and rosemary and drizzle each bowl of soup with 1 tsp oil each to serve.