Place cauliflower in saucepan and add boiling water. Cook over medium heat for 15 to 20 minutes. Meanwhile, cook potatoes the same way.
When cauliflower is tender, place in casserole dish and grate mature cheese over.
Bake in the preheated oven for 5 to 10 minutes.
Drain potatoes; mash with the butter and creme fraiche. Season with salt and pepper. Take out casserole dish from oven and spoon over cauliflower cheese. Return to the oven until golden brown, 10 to 15 minutes. Take out and serve.