Preheat oven to 180 C / Gas 4. Grease a rectangular quiche dish or tart tin and lightly dust with flour.
Roll out the pastry on a lightly floured surface and line the prepared quiche dish or tart tin, pressing down gently into the base and sides. Cut off any excess pastry and pinch the edges. Brush base with a thin coat of Dijon mustard.
In a medium frying pan, melt 1/2 tablespoon butter over medium heat. Add the spring onions and cook and stir for 5 minutes. Add bacon and cook over a low heat for 7 minutes before adding the mushrooms and cooking for a further 2 minutes. Remove and let cool down a little bit. Spoon into the shortcrust pastry case and sprinkle with fontina cheese. In a bowl, beat egg with single cream then season with salt, pepper and nutmeg and pour into case.
Bake in preheated oven for 30 minutes, until centre is set and slightly golden. Remove from oven and allow to stand 10 minutes before serving.