another vote for raw veg over cooked.
Also when my 3 were little, I used to whizz raw carrot, onion, celery, mushroom... whatever I had really... up in the processor and add to shepherd's pies. It's 'minced' so small that they can't really detect it when it's mixed well into the real mince, especially if your gravy is dark enough!
If they'll go for smooth soups and pasta sauces then you could hide a lot of things in there.
I sometimes (for my most veg phobic one) do a sort of bean bake type thing, which has onion and then things like carrot and butternut squash diced to roughly the same size as a bean, plus a can of baked beans. Can be topped with cheese & breadcrumbs or mashed potato, or served with rice.
Annabel Karmel, for all her faults, does do a recipe in her Fussy Eaters book for sweetcorn fritters. This particular recipe tells you to blitz the sweetcorn in the processor with the rest of the batter, so there are no tell tale pieces of sweetcorn for them to spot.
IME, it's a lot harder to hide green veg than tomato or root vegetables. I should also say though that my children suddenly and inexplicably got a lot better about it, so there is hope!