You can make many cream soups or chowders by starting with a roux, which is a blend of butter and flour cooked into a thick paste. You then add water or broth, and slowly simmer other ingredients, such as finely diced vegetables, meats or fish, until they are tender and the soup's thickness is to your liking. If it's still too thin, you can stir another roux in at the end of the cooking period, or add beurre manier, which is a lump of softened butter and flour that behaves the same as a roux. Whichever method you use, stir constantly until the soup reaches the consistency you want.

You can use flour or cornstarch to thicken soup by first blending it with cold water or broth. Called a slurry, you then stir the thickened liquid slowly into the hot soup. A slurry made from equal parts flour and water is also called a white wash, since the flour may alter the color of the soup or stock. It takes about 1 tablespoon of flour to thicken 1 cup of broth. Blending the flour and water thoroughly with the water minimizes clumping when the slurry is stirred into hot soup. If you are using a cornstarch slurry, it will take about 2 ounces of cornstarch to an equal amount of water of water or broth to thicken 1 quart of liquid, and the thickening action only takes effect, once the soup starts to simmer.

When using flour to thick soups, the starch molecules absorb water or other cooking liquid, causing them to enlarge. Once you expose the flour and water mixture to heat, the molecules move around until they stabilize, thickening whatever liquid they are added to. If you cook the soup too long, the starches start to break down and to lose their thickening ability. As the soup cools, sometimes, the starches thicken even more. You can remedy this only by slowly reheating the soup until it returns to its original consistency.

Cornstarch behaves the same as flour when used a thickener, but absorbs liquids much more readily and lends a clear shiny consistency to soups instead of the opaqueness that a flour thickener imparts. Cornstarch dissolves more readily in cold water or chilled broth, and is less likely to produce lumps in hot soup. You can substitute arrowroot for cornstarch, as they both produce similar results as a thickener that adds no flavor to soups. You can cook it longer, without losing its thickening ability, but you should not use it in cream soups, as it gives them a slimy texture.