Poutine en sac is a steamed pudding cooked in a cloth bag.
It is sometimes referred to as “poutine à la vapeur” (steamed pudding), because of the cooking process.
In more colloquial Acadian or Québécois French, it’s sometimes called “poutine steamée.”
The pudding will be a mixture of flour, sugar, baking powder, eggs, milk, fat (such as lard, shortening, butter or margarine), along with some sort of fruit. The mixture will be formed into a ball, put in a cloth that is tied up at the top, and placed on a rack over boiling water in a pot to cook for several hours by steam.
It can be served with cream, a light syrup sauce made of brown sugar and water, or fried salt pork.
In earlier times, the cloth bag would have been placed directly into the water.