Heat the oven to 200°C/Fan 180°C/Gas 6. Peel and grate one garlic clove into a bowl. Zest in the lemon and squeeze in half the juice. Add 2 tsp sumach and ½ tbsp olive oil. Add plenty of salt and pepper.
Make 3 or 4 deep slashes in each chicken thigh and drumstick. Rub in the sumach marinade, working some into the cuts. Place the thighs and drumsticks into a baking dish. Roast for 25 mins.
While the chicken is cooking, peel and finely chop the carrot and onion. Finely chop the celery stick. Peel and grate the remaining garlic clove. Drain and rinse the lentils.
Heat a deep frying pan. When warm, drizzle in ½ tbsp olive oil. Add the carrot, onion and celery. Cook on a low heat for 15 mins till the veg have softened. Take off the heat and set aside.
While the veg is gently cooking, finely chop the parsley leaves and stalks. Place into a bowl. Peel and finely chop the shallot and add that with 1 tbsp Dijon mustard, 1 tsp caster sugar and 2 tbsp red wine vinegar. Pour in 1 tbsp olive oil and 2 tbsp cold water. Add a good pinch of salt and pepper and mix well. This is your green sauce. Set aside.
When the chicken has cooked for 25 mins, add the cherry tomatoes to the tin. Return to the oven and roast for a further 15 mins. The chicken should be cooked through, with crisp skins.
When the chicken has 5 mins left to cook, add the grated garlic to the softened veg. Stir. Crumble in ½ the chicken stock cube and pour in 150ml boiling water. Put back on the heat and bring to a bubble. Add the lentils and gently stir. Cook for 3 mins to warm the lentils through. Add the baby leaf spinach to the lentils and stir to just wilt it.
Spoon the braised lentils on to two warm plates or bowl. Top with a chicken thigh and drumstick each. Add the roasted tomatoes. Serve with the tart green sauce and a pinch of sumach sprinkled over the top.