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Heat oven to 200C/180C fan/gas 6. Line a baking sheet with parchment. Whisk the egg whites until stiff, then fold in the salt and fennel seeds slowly to create a paste that holds its shape. Flatten about 1/4 of the paste into a circle on the baking sheet. Put the celeriac on top of the paste, then cover with the rest of the paste, making sure it is completely sealed in an even layer. Bake in the oven for 3 hrs 30 mins. To check whether it’s cooked, poke a skewer or thin knife through the crust into the celeriac. It is ready if there is no resistance. If still hard in the middle, return to the oven for 45 mins. Once cooked, rest for 1 hr still in its crust.
Once rested, cut off the top of the crust with a bread knife and keep to one side. Cut off the top of the exposed celeriac, discard and scoop out the inside, being careful not to break the celeriac skin. Mash the scooped out flesh with a fork.
Melt the butter in a medium frying pan over a low heat, until the solids in the butter start to caramelise and smell nutty. Stir the celeriac into the butter to heat through, then season. Spoon back into its shell, replace the lid of the crust and put the whole celeriac onto a board or plate ready to serve.