I stole this idea from Tesco, who have started selling hidden veg meatballs and burgers. Theirs contain onion, carrot and butternut squash, but I wanted to make a version without the sulphites.
I blitzed half an onion, a very large carrot, a small beetroot and 2 large-ish mushrooms in a food processor, to create a couscous like texture, and mixed it with around 300g of mince and some fresh herbs. It made about 28 meatballs, which is enough for a week of lunches, and some leftovers to freeze. I baked mine, but reckon you could fry them too.
I think you could make this without meat, but would potentially need an egg (or something similar) to bind the veg together.
I'm eating mine in roasted veggie Buddha bowls, which I'll add a bit of salad to before eating.