I usually sautee an onion and some garlic, and meanwhile boil or steam a variety of veg (mostly root veg) like potatoes, carrots, swede, sweet potato, broccoli, mushrooms (sauteed) cabbage, etc. etc. and then make a bachamel sauce or cheese sauce, mic the veg and the sauce, put it in a shallow dish, top with flour mixed with butter (plus breadcrumbs if using!) and oats too if you like, add grated parmesan or other grated cheese, salt and pepper, and if you want a nutty flavour sesame seeds or chopped nuts, pu tin the oven (200C) for about 30 minutes or until browned and bubbling.
Think cauliflower cheese with a twist!
Alternatively, make a tomato based vegetable sauce (like a veggie bolognese) with loads of veg cut up into it, add beans if you want (kidney beans are nice) and then top with the same crumble topping as described above.
It's not an exact science though, you can adapt the recipe as much as you want to suit your own preferences!