Prepare the vegetables: cut the celery root into 4 pieces to quarter it. Peel it and cut into 2-inch cubes. Cut the cauliflower into florets.
Place the diced celery, cauliflower, shallots, garlic and salt in a large pot and cover with water.
Bring to a boil, then lower the heat and let simmer for 20-25 minutes, or until the vegetables are tender. You can test doneness by using a knife, the celery should be soft.
Remove from heat and drain, discarding the cooking water.
Transfer to a food processor, add the butter, nutritional yeast, ground black pepper, and a pinch of nutmeg. Process for 3-5 minutes, or until you get a smooth purée. Taste and adjust salt and pepper if needed (I ended up adding about 1/2 tsp salt).
Serve immediately or reheat gently in a saucepan with some almond milk to prevent it from drying. You can also reheat it in a double boiler.
Celery purée will keep for up to 4 days in the refrigerator.
Celery and cauliflower can be steamed instead of boiled if you prefer.