Preheat the oven to 200 C ( 180 Fan) Gas Mark 6. Mix all the marinade ingredients together in a bowl.
Spread the onions out in a deep roasting tin and scatter over the garlic cloves. Put the chicken legs on top, skin side up, together with the sweet potato and cauliflower florets.
Pour over the marinade and turn the chicken legs several times until fully coated. Top the whole dish with the lemon slices.
Bake for about 1 hour or until the chicken is golden and cooked through. Serve the chicken and vegetables immediately with extra harissa.