Bring wine to the boil in a large saucepan over high heat, add mussels, cover with a lid and cook, shaking pan occasionally, until mussels open (3-5 minutes). Remove from pan and, when cool enough to handle, remove mussel meat from shells and set aside. Strain cooking liquor through a fine sieve and reserve.
Sauté onion and speck in oil in a large saucepan over medium-high heat until onion is tender (5-7 minutes). Add thyme and stir for 1 minute, then add celeriac and potatoes, and stir to combine. Add stock and cooking liquor from mussels, and simmer rapidly until vegetables are tender (about 20 minutes).
Add mussels to hot chowder, season to taste and stir to combine. Divide among bowls, top with crème fraîche, parsley and a good grind of pepper and serve with crackers.