Sweet potatoes?
Grate them and add them to everything including the meatballs & the sauce.
I made really nice veggie meatballs recently out my head. Grate sweet pots, carrot, add the veg she will eat and tell her that's all that's in them! plus soften some onion (tiny pieces) breadcrumbs and a bit of egg. Oregano. Fried up in oil and butter so they colour nice. Once added to a tomato sauce she may not even know it isn't beef 
Red onion is often more palatable - learn how to chop it tiny and cook it in oil and butter then add to whatever. (I half through stem. Cut off the pointy end, keep the hairy end. Make thin slices into the half toward the hairy bit. Turn it and slice across these cuts making tiny bits. Sharp knife essential)
Egg fried rice? I use up everything in the fridge from carrot to mushroom, peas and sweet corn, spring onion and peppers. Chop them TINY. Soften. Add cooked rice with a chicken stock cube. Beat an egg and stir it in. Cover in hoisin !
Skin and deseed tomatoes and add fresh to tinned when making sauces. (Put them in boiled water if you're not sure).
YES to red lentils. A small handful in all your sauces and blend.
Use the broccoli stem and cut as small as the peas and add. To everything she eats with peas.
Cucumber - remove the skin and seeds?
American pancakes with grated apple or blueberries a fave at mine.
Make roast pots, sweet pots, butternut and parsnip - make cubes of identical sizes then roast.
I roast with sea salt flakes to crisp them plus rosemary and thyme chopped tiny and sprinkled.
Try using butter to cook things in because it gives a light flavour but also a palatable colour which can hide what you've actually made !