Fry half of the bacon in a pan over medium heat until golden-brown and drain on kitchen paper.
Steam clams in a lidded pan with white wine for 2-3 minutes until all have opened.
Strain the clams, reserving the liquor, and mix the clam meat with pepper, parsley, and cooked bacon.
Fry onion, garlic, and remaining bacon in 25g butter, then add parsnips and thyme.
Add the reserved clam liquor, boil until reduced by one-third, then add chicken stock and simmer until parsnips are tender.
Stir in the double cream and blend the mixture until smooth.
Pass soup through a sieve, return to heat, season, and whisk in the final 25g of butter.
Ladle soup into bowls around the clam mixture and garnish with a slice of toast.