I don't like calling beans "leftovers" because a lot of us cook beans to have on hand long after they've been made. For an easy casserole, layer cooked beans, cooked greens (like chard or dandelion greens) and then top with panko crumbs dotted with butter. I think I use some leftover poached chicken I had on hand for this version. Bake it at about 350F for 40 minutes.