Depends on your location, I’m in Oklahoma and I feed High protein Game bird feed to my breeders and hatchlings. 28% protein, I offered crushed oyster shell free range for breeders, and as said above I supplement with kale out of garden.
I attach cuttle bone to the side of the cage for them to spend time eating for the calcium (this is the cuttlebone sold for use in parrot cages, like at any pet store or even Wal-mart pet aisle).
You should provide the quail with grit to help them to digest anything besides feed. I use "chick grit" sold in Tractor supply or other farm/feed store for use with chicken chicks - this is much smaller than the regular chicken grit. Also, you can buy parakeet grit (same place you would find cuttlebone), but that is more like sand. I've been known to mix both and they can take what they need. I use the parakeet tube feeders attached to the side of their cage.
I will give them small amounts on occasion of tiny dice apples, carrots, strawberries. Sometimes, I'll just do a quick grind in a small blender so it's like a chunky combo mash, and sprinkle a small amount of millet seed on top. My quail adore cabbage for some reason - they love it.....I just take a leaf and chop it into very small pieces.
High protein feed, a calcium source. Everything else is just bonus but make sure sand or grit is available.
Mine like greens, seeds and whatever bugs find their way into their enclosure. I’ll try different fruits and veg as I go.
One of my hens was desperately trying to reach some leaves just outside the aviary today so I picked some long grass, dandelions and clover for them and they enjoyed rifling through it.