Prepare the soaking water. Stir 1 tablespoon of lemon juice into 500 ml of cold water in a large bowl.
Peel the celeriac. Cut it into quarters, trim the knobbly root end, and peel away the thick skin until only pale, cream-colored flesh remains.
Cut into matchsticks. Slice into thin plates about 3 mm (1/8 inch) thick, stack two or three, and cut into 3 mm matchsticks. Drop them into the lemon water as you go to keep them pale.
Drain and dry. After a few minutes, drain in a sieve, press out the water, and pat dry on a clean tea towel until no water clings. The dressing sticks better to dry matchsticks.
Make the dressing. In a large bowl, stir together the mayonnaise, crème fraîche, mustard, lemon juice, capers and dill until smooth and evenly pale.
Toss in batches. Add the celeriac in two or three batches, mixing after each, until every matchstick is lightly coated. Season with pepper.
Chill. Cover and refrigerate for 30 minutes. The celeriac should soften slightly but still crunch when bitten, and no liquid should pool in the bowl.
Taste and adjust. Add a squeeze of lemon, pepper or a pinch of salt only if it needs it.
Plate. Mound the remoulade on four plates, drape the smoked salmon beside or over it, and finish with dill and a lemon wedge.