Vichyssoise (cold leek-based Summer soup)

Ingredients: 1-1.5kg leeks, white only 750g peeled potatoes 1-2 peeled & chopped onions 500ml chicken on vegetable stock Bouquet garni or herb mix (flat-leaf parsley, bay leaf, celery salt…) 250ml double cream Salt & Pepper (white pepper if possible, to keep it nice & white, but some prefer the black pepper speckles, so as you…