Heat 1 tbsp oil in a large saucepan over a low heat. Add the leeks, thyme and seasoning. Cover and cook for 15 mins until softened, adding a splash of water if the leeks start to stick. Add the butter beans with the water from the cans, the stock and mustard. Bring to the boil and simmer for 3-4 mins until hot. Blend the soup in a food processor or with a stick blender, stir through the parsley and check the seasoning.
Put the bacon in a large, non-stick frying pan over a medium heat. Cook for 3-4 mins until crispy, then set side to cool. Add the remaining 1 tsp oil to the pan, and tip in the kale and hazelnuts. Cook for 2 mins, stirring until the kale is wilted and crisping at the edges and the hazelnuts are toasted. Cut the bacon into small pieces, then stir into the kale mixture.
Reheat the soup, adding a splash of water if it is too thick. Serve in bowls sprinkled with the bacon & kale mixture.