Heat the oil in a large saucepan on medium heat and add the chopped onions. Season with salt and pepper and cook for 5-6 minutes, stirring now and then until they start to soften.
Next, add the diced celeriac, apple, and potato. Stir well and cook for a couple of minutes.
Add the vegan stock, dried thyme, bay leaves, and nutritional yeast. Stir well, bring to a gentle simmer, and cook for 20 minutes.
After 20 minutes, stir in the lemon zest and plant-based milk. Start with half of the zest and build from there depending on how much lemon flavour you want to come through. Cook for another 5 minutes or until the celeriac is tender. If necessary, add a little more water or stock.
Once the celeriac is cooked through, turn off the heat, remove the bay leaves, and blitz until smooth using a hand blender (you could also transfer to a larger, more powerful blender).
Adjust the seasoning and serve with homemade vegan cream, a squeeze of lemon juice, and chopped fresh parsley. This is also delicious when topped off with crispy tofu pancetta.