In a large pot over high heat, add clams and wine then cover pot with lid and steam until clams open (discard any that don’t).
When cool enough to handle, remove clam meat from shells.
In a food processor, add half the clam meat and pulse to a paste. Finely chop remaining clams then combine with puréed clam meat and set aside.
In a pot over medium heat, melt butter then add flour and cook, stirring continuously, for 5 minutes. Add milk slowly, stirring continuously to avoid lumps, then gradually add stock and sherry. Bring to the boil then reduce heat to low and gently simmer for 10 minutes, being careful that it doesn’t catch on the bottom of the pot.
In a separate pan, heat oil over medium heat. Add onion and garlic and cook, stirring regularly, until very soft and translucent. Stir in nutmeg then add to roux mixture along with the clam meat. Bring back to the boil then stir in parsley and season to taste with salt and pepper.
Combine mascarpone and Indian seasoning.
To serve, divide chowder evenly between soup bowls and finish a swirl of Indian-spiced mascarpone and picked parsley leaves.