Peel and dice the onion and garlic. Clean and slice the leek.
Sweat the onion, leek and garlic in a large pan over a low-medium heat for 10 minutes while you prepare the other vegetables
Peel and dice the potato and celeriac. Clean thoroughly
Add to the pan and sweat for 2-3 minutes, stirring frequently
Add enough stock to the pan to cover all the vegetables with about an inch extra of water.
Bring to the boil and simmer gently for 30 minutes, stirring occasionally. Pre-heat the oven to 190C
Top up with water or stock as necessary and cook until the potato and celeriac is soft and almost falling apart.
Blend with a stick blender, season with salt and pepper and cook for a further 5-10 minutes
Meanwhile, toss the bread cubes for the croutons in the olive oil
Lay the bread cubes out on a baking tray and sprinkle with blue cheese
Cook for 5-10 mins till the croutons are crispy and the cheese has melted
Check the seasoning of the soup and serve with a teaspoon of creme fraiche stirred through each bowlful and with a garnish of croutons and chopped fresh chives.