For the spice rub, mix spices until well combined. Pound garlic, thyme and salt using a mortar and pestle, then add spices and olive oil to create a paste. Rub paste all over chicken and refrigerate for 2 hours.
Preheat oven to 180°C.
Rinse lentils and place in a large saucepan with onion, carrot, bay leaf and salt. Pour in chicken stock and 1 cup water, bring to boil and simmer for about 25-30 minutes; the lentils should be tender but still hold their shape. Allow to cool, then drain and discard bay leaf. Whisk together oil and vinegar in a bowl, then mix through cooked lentils. Check seasoning.
In an ovenproof frying pan, heat 1 tbsp oil on a medium heat and seal chicken, about a minute each side. Place in oven and roast for 20 minutes, until just cooked. Remove and allow to cool.
For the dressing, whisk together the mayonnaise, mustard and seasoning in a bowl. Slowly whisk in the verjuice until the dressing is of pouring consistency.
Slice the chicken into thick diagonal pieces. Fold celery, rocket, parsley and lemon zest through lentils. Plate lentils with the chicken and drizzle with dressing.