In a large bowl, combine the flour, baking powder and salt. Combine the egg, butter, water and lemon juice; stir into dry ingredients until a soft dough forms. Spread half of the dough into a greased 13-in. x 9-in. baking dish.
In a large saucepan, melt butter. Stir in flour and salt until smooth; gradually add the milk. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Stir in turkey and mushrooms. Pour over biscuit dough. Drop remaining dough into nine biscuits onto turkey mixture. Bake at 425° for 15-20 minutes or until biscuits are golden brown.