Onions: For this recipe, we prefer to use frozen diced onions for two reasons. Of course, it saves the step of chopping the onions yourself. More importantly, however, the frozen onions break down better and become softer in the slow cooker, especially when cooking this soup quickly on high or for only six hours on low. You can certainly use fresh onion, chopped to equal 2 cups, if you would prefer your onions to be a bit more firm and crunchy in the final soup.
Cream of Mushroom Soup: When selecting a cream of mushroom soup, read the labels carefully, as ingredients and nutritional profiles vary widely. We recommend choosing an organic, lower fat brand such as Health Valley.
Make-ahead options: You can mix all ingredients except chicken together the night before cooking, then refrigerate overnight. Then just pour the refrigerated mixture into your slow cooker in the morning. We recommend adding the chicken in the morning when you begin cooking.
This soup makes a large batch, and refrigerates well to be reheated for additional meals later in the week. It also freezes well after cooking and can be frozen in a tightly sealed baggie or freezer container.