Prepare the pearl barley and cook according to pack instructions. Set aside to cool slightly.
Meanwhile, roughly crush the fennel seeds. Mix the chicken thighs with the crushed fennel seeds, lemon zest and 1 tbsp lemon juice, ½ tbsp olive oil and the garlic. Season and leave to marinate for 20 minutes.
Heat a griddle pan over a medium flame, then add the chicken thighs. Cook for 5 minutes on one side, then turn over and cook for a further 4-5 minutes on the other side until the chicken is cooked through and no longer pink inside. Remove from the heat, cover and leave to rest.
Add another ½ tbsp olive oil to the griddle pan, then add the leeks. Grill the leeks for 2 minutes on each side until they start to create grill lines. Remove and set aside.
Whisk together the honey, mustard, vinegar, remaining oil and remaining lemon juice until combined. Shred the chicken into thin strips. In a large bowl, toss together the pearl barley, chicken, leeks and dressing, along with the chopped mint and peas. Serve.