I would steam them for longer. I wouldn't cook them in water because a whole lot of the nutrition goes into the water, which you then throw out. I agree that canned beans are very low in nutrition and there are also grave concerns about what's in the can material that seeps into the foods. With fresh beans, the longer you cook them in water, the fewer nutrients they have. That doesn't make any sense if you are going to the trouble of making your own cream of mushroom soup, and then you use canned beans.
You could also use frozen beans, which are frozen right at the point of harvest and are nutrient-rich (at least at much as anything grown commercially these days!). Maybe if you used the French green beans which are sliced, they would cook faster and then be more soft for your casserole.