Your question is an interesting one. cooking can destroy some nutrients but also can make others available to humans that digestion alone can not release. Casserole isnt really the issue, its heat and time, and cooking is about creating a desirable result not just making fuel so yes it may reduce for example the value of a carrots nutrition but a lovely braised carrot is delicious in a different way to a raw one.
Many people only eat cooked food - plenty of kids wont touch salad, and as a rule given enough food to fill our bellies most of the time we can afford to be suboptimal on our intake - sacrificing some nutrient for taste is a working compromise.
enjoy food
S