If fresh beans are not used for the casserole, then frozen beans are the second best option. They have already been par-cooked, so you need to defrost them, mix them with the sauce ingredients, and bake. There are fresh canned green beans, but they will have a mushier texture and saltier flavor. They are fully cooked and have been high-temperature processed in a retort, which will make them much softer. You'll need 4 cups of frozen or canned for the recipe.