I have looked after lots of vegetable dodging children, most have accepted that if they cannot see it or really obviously taste it then they do not mind veg I hide in their food. Small amkunts can be subtle but they add up and are better than nothing.
Blended celery and broccoli and cauliflower in a cheese type pasta sauce. We started with just celery and added the other vegetables gradually. Broccoli will always be visible, and too much will be tasted.
Spinach blended into pancake mix (sounds gross but you cant taste it and it makes them a fun green colour) can do the same with beetroot if your feeling adventurous - I think you can taste it a bit but my kids happily eat 'vampire pancakes'.
I find it very easy to hide veg in all types of curry sauces. Especially onion, celery, carrots because when they are cooked and blended you dont notice them at all. Then stuff like cauliflower, broccoli, extra tomatoes, peppers, are easily covered by spices. Throw a ton of spinach in anything spicy as it tastes of nothing and chopped up it disappears quite well. Then once the sauce is done stir in whatever cooked meat they like.
Things like potatoes, sweet potatoes, squash and parsnips are good to mash and add thickness to sauces or a soup or stew, especially gravy ish type meals. Things like lentils and chickpeas will often really change the texture so are more noticable
I boil or steam a ton of random blandish veg, mash or blend it down, sometimes sorted into colours and then freeze it in ice cube trays- then chuck a few into whatever I am cooking. Garlic or lemon covers most things.
Anything new that I dont blend, I might crush or cut up so fine its not visible in the meal. Things like onion and very finely chopped celery and carrot can be mixed into hamburger meat- especially if they like ketchup its not noticable. Same goes for a chilli or bolognaise, shepards pie, sausage meat if you make patties or sausage rolls or pasties.
Sometimes novel forms helps as an introduction to a new flavour, one girl started liking onion from first having onion rings- learning to like the taste, and now enjoys onion in lots of food. One boy first had sweetcorn when we made corn 'ribs' with bbq sauce but he now eats corn on its own.
Making vegetables taste better with things like butter doesn't take away the benefits of the vegetable. It may be a less healthy dish overall but the vegetables they eat will always be better for them than the vegetables they don't touch- extra stuff doesn't cancel out the minerals. So currently the children I look after will accept actual vegetables like peas (with cheese), carrots (honey glazed), parsnips (roasted in fat) and broccoli (in a cheese sauce) and green beans (on their own).