Substitute for Canned Soup

The standard substitute for canned soup in recipes like casseroles which call for cream of chicken or other condensed soups is found in More with Less Cookbook by Doris Longacre. She suggests using a medium-thick basic white sauce. The directions are:
Melt in a heavy saucepan 3 Tbsp. butter. Blend in 3 Tbsp. flour and 1/4 tsp. salt. Using a wire whisk to prevent lumps, stir in milk, stock, or a combination equal to one cup of liquid. To make a substitute for chicken soup, use part chicken stock or bouillon. To make mushroom soup use 1/4 cup chopped mushrooms and 1 Tbsp chopped onion sautéed in the butter before adding the flour. Once the liquid has been added, cook until smooth and thickened. This amount substitutes approximately for a 10 oz. can of condensed soup.

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That was the "standard substitute" and it works well. My version is a bit different. In an effort to cut down on fat AND because it's easier, I simply whisk the flour into cold liquid (milk/flour/stock/etc.) and bring it to a boil. After it boils, I continue to cook it for 2 minutes to make sure the flour is cooked well. This means I don't use butter in my recipe. I also omit the salt since I usually use a bouillon cube rather than stock.

For a more creamy soup, I use a full cup of milk and add a bouillon cube (1/2 cube per 1 cup of liquid if using Knorr) rather than using part water.

I prefer to use whole wheat flour rather than white plain flour. The whole wheat flour doesn't thicken it as much, but I have not found that to be a problem in casseroles.