Fill a medium pot with water and place it over moderately high heat; add 2 teaspoons of salt to the water. Once it reaches a boil, add the celery root and cook for 10 minutes. At the 10 minute mark, add the potato, and cook for an additional 10 minutes.
While the celery root and potato are cooking, make the dill oil. To a food processor, add the dill and olive oil; pulse until the dill is broken up into little bits. Transfer to a glass jar and allow to sit while you finish the celery root mash.
After cooking for 20 minutes, the celery root and potato should be tender when poked with a fork. Turn off the heat and drain the celery root/potatoes in a colander and carefully add them to a food processor, along with the garlic cloves; pulse until smooth. Note: You may have to do this in batches, depending on the size of your food processor.
Add the pureed potato and celery root back to the pot and turn the heat to low. Mix in the butter, cream, celery salt and pepper. The mixture should be thick and smooth(ish). Salt to taste. I ended up adding an additional 1/2 teaspoon of salt.
If you like (I did), run the dill oil through a small sieve, catching any dill leaves and discarding. The oil will be a pretty bright green. Add the celery root mash to a bowl and pour the oil atop, along with a few fresh dill leaves. Serve immediately.