Melt the butter in a large saucepan over medium heat. Cook and stir the onion and garlic in the butter until soft but not brown. about 5 minutes.
Pour the chicken stock into the pan and whisk in the tomato puree, double cream and lemon juice. Turn the heat to high and bring the soup to the boil. Turn the heat down to medium-low, add the 360g pecan pieces and the chopped chipotle chillies and simmer the soup until the pecans soften, about 30 minutes.
Set aside 500ml of soup. Pour the rest of the soup into a liquidiser, filling the jug no more than halfway full. Hold down the lid of the liquidiser with a folded tea towel and carefully start the liquidiser, using a few quick pulses to get the contents moving before leaving it on to puree. Puree in batches until smooth and pour into a clean pan. Alternately, you can use a hand blender and puree the soup right in the cooking pan.
Stir the reserved 500ml of soup back into the pot and bring to a simmer and season with salt to taste. Serve, hot, with a sprinkle of chopped pecans for garnish.
Tinned chipotle chillies in adobo sauce can be purchased in Hispanic speciality shops or online.