Lightly bash the rosemary sprigs with a rolling pin to help release the powerful aroma and flavour when cooking.
Heat the oil in a saucepan and stir in the potatoes and rosemary. Cover with a lid and cook on a low heat for 4-5 minutes. This will create some steam in the pan, preventing the potatoes from sticking to the base.
Stir in the leeks and garlic, then cook for a couple of minutes until beginning to soften. Add the stock and bring to a simmer, leaving it to cook for 10 minutes.
Remove from the heat, take out the rosemary sprigs and stir in the gorgonzola cheese and Dijon mustard. It doesn't matter if the cheese doesn't melt completely as you now need to place the soup into a blender or food processor and blitz until smooth.
Season with salt and pepper, making sure you taste the soup first as gorgonzola is quite a salty cheese. If the consistency seems too thick, add some extra vegetable stock to loosen.
Serve with some fresh bread while the soup is still hot - walnut bread is really nice - or if you're not eating it straight away, gently reheat, being careful not to let it boil as this will give the soup a grainy texture.