Discard the outer leaves of the cabbage, or save them for another dish. Cut the stalk from as many of the paler leaves as you will need to line four metal rings of 7or 8cm diameter. Cook the leaves in boiling water until tender, then refresh in cold water.
Peel the celeriac and slice the flesh into pieces 1/2cm thick. Boil these for two minutes, then transfer to a baking sheet, brush with olive oil and roast in a hot oven for five minutes.
Melt one tablespoon of butter in a wide pan and cook the chopped leeks over high heat with the garlic and thyme. When the leeks are tender, if there is a lot of liquid in the pan, pour it off before continuing. Add the mustard and cream and continue cooking for five minutes more. Cool and then add the eggs and chestnuts. Season with salt and pepper.
Line four metal rings with the cabbage leaves so there is a single layer all round and enough overlapping to cover the top. Line the base with celeriac, then press in a layer of the chestnut mix. Repeat this, then finish with a layer of celeriac. Fold over the cabbage leaves and press gently.
Brush the tops with olive oil and cook for 15-20 minutes in a moderate oven until the timbales are firm, occasionally moistening with a splash of stock or water.
Heat the butter in a wide pan and fry the shallots, garlic, thyme and mushrooms for five minutes or so, until the mushrooms are almost tender. Add the cider and simmer for five minutes, then add the cream and continue simmering for a few minutes more, until the sauce has thickened a little. Season with salt and pepper.
To serve, turn the timbales onto a plate, bottoms up, and spoon some mushrooms over each one.