Season the chicken thighs with plenty of pepper and roll in the thyme leaves. Heat the oil in a large, heavy-based frying pan and cook the chicken thighs over a medium heat for 20 minutes until cooked through, turning frequently for the first 10 minutes, then covering with a lid for the final 10 minutes.
Add the lemon rind and juice to the pan and toss with the chicken. Mix the mustard into one of the cartons of crème fraîche and then add both cartons of crème fraiche to the pan with the spinach leaves. Toss and heat for 2–3 minutes until the spinach has wilted and the sauce is hot.