Put a large frying pan on a medium-high heat and add enough olive oil to cover the base. Fry the cubed potatoes for about five minutes, stirring regularly, until lightly golden all over, then add the garlic and onions, and fry, stirring often, for five to eight minutes more, until softened.

Spoon the potato mix on to a plate, then put half the butter and all the sliced mushrooms in the pan. Add most of the thyme leaves, season generously, then stir in the allspice and fry, stirring occasionally, for 10 minutes, until the mushrooms release their liquid and start to caramelise.

Return the potato mix to the pan, stir to combine, then take off the heat. Stir in the cream cheese and put to one side.

Heat the oven to 200C (180C fan)/390F/gas 6, and grease a 30cm x 25cm roasting tray. Line the tray with most of the filo sheets, so they drape over the sides, and drizzle olive oil between each layer. Spoon on the filling, spread it out into an even, not-too-thick layer, then fold over the overhanging filo so the filling is completely enveloped. Drizzle with more olive oil, then bake for 35 minutes, until golden and crisp all over.

Just before you take the pie out of the oven, melt the remaining butter in a small pan with a few thyme leaves, then stir in the Marmite. Slide the bougatsa on to a serving board, cut it into four, pour over the Marmite butter and serve straight away.