(Nutrition information is calculated using an ingredient database and should be considered an estimate.)
Heat the oven to 180C/160C fan. Heat the Flora in a small saucepan over a medium heat, then add the onion along with a big pinch of salt, fry until soft and golden, around 10 mins, then add the garlic and cook for 2 mins further. Add the cream, milk, garlic, thyme, bay and dijon, bring to a gentle simmer then leave to infuse.
Stack the celeriac and potato slices in a baking dish, starting with 1/3 of the slices. Pour over 1/3 of the cream mix, then repeat with the rest of the mixture. Sprinkle with the parmesan and bake for 30 mins, until the vegetables have no resistance with a skewer is pressed through and the top is golden. Allow to sit for 10 mins, then serve.