My budgies ate primarily seeds for their whole lives. I had no luck getting them to eat any type of fruit or veggie until one day I was cutting into a bell pepper and I saw the seeds. The thought occurred to me that they would be more likely to recognize the seeds as a food source than just the pepper part.
For the first week, I took bell peppers, shaved the seeds off the core and sprinkled them on top of their seeds every morning. When they consistently ate the seeds, it was time to move on to the second step. The second step was taking the middle on the pepper and putting in the cage. They recognized the seeds and ate them right away. They also ended up chewing up the pepper part. Now they will just eat the pepper part even without the seeds attached.
Unfortunately, my older birds would not go up the the pepper or their seed cup to investigate the white seeds. They seemed afraid of the pepper and would not touch the seeds. After observing them ignoring and staying as far away from the the peppers as possible for a week, I took a slice of the bell pepper with the fresh seeds on it, coated it in bird seed and placed it in an emptied seed cup. This made it so in order to eat the seeds that they love, they had to touch the pepper and could no longer avoid it. This was done in the hopes that it would allow them to associate the bell pepper and it's taste a positive experience since to animals, food=positive. Now I can put the bell pepper in a filled seed cup and they prefer to eat the seeds off of the pepper. They are not the best pepper eaters yet but they eat a little more of the pepper every time!
**Be sure to replace the bell peppers frequently**