Heat a frying pan over a medium heat and fry half of the bacon for 1-2 minutes, or until golden-brown. Remove from the pan, using a slotted spoon, and set aside to drain on kitchen paper.
Meanwhile, heat a lidded frying pan over a medium heat. Add the clams and white wine, cover the pan with a lid and steam the clams for 2-3 minutes, or until all of the clams have opened. (Discard any clams that have not opened during cooking.)
Strain the clams through a colander and reserve the cooking liquor. Pick the clam meat from the shells, place into a bowl, season, to taste, with freshly ground black pepper. Stir in the parsley and cooked bacon and set aside.
Heat half of the butter in a saucepan until foaming and fry the onion, garlic and the remaining half of the bacon for 2-3 minutes, or until softened. Add the parnsips and thyme and cook a further 2-3 minutes. Pour in the reserved cooking liquor from the clams, bring the mixture to the boil and cook until the volume of the liquid has reduced by one-third.
Pour in the chicken stock and return the mixture to the boil. Reduce the heat and simmer for 8-10 minutes, or until the parsnips are tender.
Stir in the double cream and blend the mixture in a blender until smooth.
Pass the blended soup through a sieve into the saucepan and return to the heat. Season, to taste, with salt and freshly ground black pepper and whisk in the remaining 25g/1oz butter.
To serve, spoon some of the clams into the middle of each of 4 serving bowls. Ladle the soup around the clams and top with a slice of toast.