Stir in the mustard mayonnaise, starting with 1/2 cup, then progressively add more until you reach the desired consistency. This will depend on how much celeriac you have, and how 'saucy' you like your remoulade!
Season to taste, and add some herbs if you like.
Mustard Mayonnaise
Whisk the egg yolks with a little salt and the lemon juice in a small basin with a non-slip base, or rest the bowl on a damp tea towel so that it doesn't spin around.
Once smooth, add a couple of drops of olive oil to the egg yolk mixture. Continue to whisk, then add the remaining olive oil, whisking all the while.
Whisk in the mustard, then season to taste.You may wish to add more lemon juice or salt.
Notes
The mayonnaise can be made a day ahead. The remoulade will keep for up to three days in a sealed container in the fridge.