I've c+p this from an old thread:
Peeled a whole celeriac, then used the food processer to slice it up. (I have two different blades, and I used the one that does the thickest slices, but they were still quite thin).
Piled all the slices into a casserole dish.
Crushed four cloves of garlic into it, added salt and black pepper, and then a 150ml of double cream.
You actually need twice as much liquid, so I used the tub to measure the same quantity of milk (semi-skimmed). Ideally, in terms of the carb count, it would probably have been better for me to use 300ml of single cream, but I didn't have any to hand.
Stir everything round so that the garlic gets stirred into the slices of celeriac.
Then I put the lid on the casserole and cooked it for 5 minutes on high in the microwave. Took it out and stirred it, then another 5 minutes on high.
Then bake in a hot oven (c. 200) for 20-30 minutes, till it's cooked through.
Take the lid off and grate some cheddar cheese all over the top (as much as you like), then back into the oven without the lid until it's nicely browned.
That made enough for 4 people. Not that anyone else in this house is getting their hands on it!