Layer the potatoes and celeriac in a buttered gratin dish with the garlic, seasoning with salt and pepper as you go.
Pour in enough hot stock to come up nearly to level of the top layer.
Cover with greaseproof paper and bake for 45 minutes.
Mix the creme fraiche and mustards in a bowl.
Spread over the top of the gratin. Return to the oven and bake for another 10-15 minutes until the celeriac and potatoes are tender. (Do not add the cream mixture before step 6, as mustard turns bitter when cooked for too long).