Peel the celeriac roots, cut in half and blanch them.
Prepare the stuffing as follows: Cook the mushrooms and shallots in butter for a few minutes, stir in the flour and cook a further few minutes.
Add tomato puree, about 1 cup, seasoning, and stir well.
Scoop out the centres of the celeriac halves and fill with the mixture.
Arrange them in a buttered1 oven dish, dot with butter, cover and bake in a slow oven for 45 mins.