Heat oven to 200C/180C fan/gas 6. Tip the beetroot, celeriac, onions and whole garlic cloves into a shallow roasting tin. Add the oil, 1 tbsp thyme, half the lemon zest, fennel seeds and a squeeze of lemon juice, then toss together. Roast for 20 mins while you prepare the chicken.
Stir the mustard powder and paprika into 2 tbsp yogurt in a bowl. Add half the crushed garlic, the remaining lemon zest and thyme, and juice from half the lemon. Add the chicken and toss well until it’s coated all over. Put the chicken in the tin with the veg and roast for 40 mins until the chicken is cooked through and the vegetables are tender.
About 5 mins before the chicken is ready, wash and drain the spinach and put it in a pan with the remaining crushed garlic. Cook until wilted, then turn off the heat and stir in the remaining yogurt. Scatter some extra thyme over the chicken and vegetables, then serve.