Heat oven to 200C/180C fan/gas 6. Grate the zest and squeeze the juice from the lemon into 2 bowls, then push the wedges inside the cavity of the chicken with the parsley stalks. Put the chicken in a roasting tin and roast for 30 mins.
Meanwhile, scrub the sweet potatoes (no need to peel) and cut into chunks. Put in another roasting tin with the onions, fennel, and 6 whole garlic cloves, then toss with the oil. When the chicken has had its 30 mins, put the potatoes in the oven and roast for 30-40 mins more.
Meanwhile, finely chop the remaining garlic clove for the gremolata, then mix with the lemon zest and parsley. Mix the lemon juice, oil and some black pepper in a bowl.
Remove the chicken and potatoes from the oven. Cover the chicken and leave to rest. Stir half the gremolata mix into the lemon juice and oil. Stir the spinach through the potato mix and return to the oven for 2 mins to wilt. Tip into the lemon dressing and toss well. Carve a chunk of breast meat from the chicken (you need about 140g), remove the skin, then cut into pieces. Toss into the salad with the pomegranate seeds, then serve scattered with the remaining gremolata.