Heat a big pot of water over high heat.
Peel the celery root by cutting off the outer layer of it with a knife.
Then dice the celery root into small-medium sized pieces.
Once the water is boiling, place the diced celery root into the pot.
Let it cook in the boiling water for 25 minutes.
Drain the celery root and put it into an empty pot.
Add the coconut milk to the pot.
Using a masher, mash the celery root well, making sure to mix it with the coconut milk.
Add the salt, pepper, ground mustard seed, garlic powder, and lemon juice to the mashed celery root.
Note: I like a lot of mustard flavor in my celery root, so if you are not sure, maybe start with 1/4 tsp of mustard seed and work up to 3/4 to see how you like the flavor.
Chill or eat warm. I prefer it cold.