Place the chicken breast in a medium pot and cover with plenty of cold water. Season well with salt and bring to a gently simmer. Cook for 8 minutes. Turn off the heat and allow to completely cool in the cooking liquid. Remove and set aside.
Meanwhile, cook the pearled barley in a pot of boiling water until tender, around 15-20 minutes. Drain and set aside
Rinse the quinoa thoroughly and cook in a pot of boiling water until tender, around 10 minutes. Doing the same with the millet, but cooking it for around 15 minutes. Drain both and set aside to cool.
Pod the broad beans and blanch them in a pot of boiling water for 2-3 minutes. Run the beans under cold water and then remove the touch outer skin. Place the now double podded beans in a large bowl along with the cooked and cooled barley, quinoa and millet. Shred the chicken and add to the bowl along with the mint, dill, parsley, olives, radishes and walnuts. Mix so that everything is well incorporated.
For the dressing, whisk all of the ingredients together, season with salt, check for acidity, adding more lemon if needed. Drizzle over the salad, toss to coat and serve.