Mix the flour, salt and suet together then add sufficient cold water to form a soft dough. Turn out onto a lightly floured surface and roll out. Use 2/3 of the dough to line a greased 2 pint pudding basin, reserving the remaining 1/3 for the lid. Seal any gaps well. Mix the onion, mushrooms and bacon together, season, and put into the centre of the pudding, adding a little stock or water. Add the dough lid, pressing the edges firmly together and sealing with a little water. Tie a circle of greaseproof paper over the basin and cover securely with kitchen foil. Place on a stand in a saucepan, and pour round sufficient boiling water to come about half way up the basin. Cover and boil for 2 to 2 1/2 hour's, topping up with boiling water as necessary. Serve with potatoes and a green vegetable.