Have you tried putting mealworms in the salad dish? That usually gets them to eat their greens. Of course you'll have to keep an eye that the lil buggers don't wriggle out and bury themselves beneath the substrate
Another one to try is rocket, mine LOVES rocket. I think a lot of beardie owners find that their beardie enjoys a heaping handful of rocket
OR..... Raspberries, the colour of them is soo bright that they can't not notice them, also they seem to love them. Chuck a few chopped up within the salad and see what it does, not too many though and you can't use raspberries all the time as they're very acidic and watery....not good for a beardie.
If they don't work let us know, theres always someone out there with another idea