Preheat oven to 200ºC . Grease and lightly flour large baking sheet. Combine water, half cup margarine and salt in saucepan. Bring to boil over medium heat. Add flour all at once. Stir vigorously with wooden spoon until smooth and dough forms a ball and leaves side of pan. Remove from heat. Add eggs, 1 at a time, stirring until smooth after each addition. Spoon dough into large pastry bag fitted with fluted nozzle. Pipe dollops of dough, about 1 tablespoon each, onto prepared baking sheet, leaving room for spreading. Bake 20-25 minutes until golden brown. Let cool. Melt 2 tablespoons butter/margarine in a large skillet. Add onions and mushrooms; saute over medium-low heat until tender. Stir sour cream into onion mixture; let cool. Cut baked pastry swirls in half horizontally. Using a spoon or small paring knife, remove any soft dough inside puffs, leaving a hollow, 2 part shell or crust about quarter to 1 cm thick. Spoon about 1 tablespoon sour cream mixture in bottom portion of each swirl. Replace top portion of swirls. Preheat oven to 180ºC . Bake on large baking sheet 10 minutes or until heated through.