Preheat the oven to 200/400F/Gas 6. Place the syrup, lemon zest and lemon juice into a bowl and whisk until well combined. Add the chicken pieces and turn until they are completely coated in the mixture. Season well with salt and freshly ground black pepper and set aside for at least 10 minutes to marinate.
Heat 40g/1½oz of the lactose free butter and half of the olive oil in a pan over a medium heat. When the lactose free butter is foaming, add half of the marinated chicken pieces and fry for 5-6 minutes, turning occasionally, until golden-brown. Set the chicken pieces aside on a warm plate.
Repeat the process with another 40g/1½oz of butter, the remaining oil and the remaining chicken pieces. When the chicken pieces are golden-brown, set them aside.
Add the lemon slices and residual marinade juices to the pan and stir well, scraping any burned bits off the bottom of the pan with a wooden spoon.
Return the cooked chicken pieces to the pan, then add the hot chicken stock and the thyme sprigs and stir well.
Bring the mixture to the boil, then place into the oven to cook for 30-35 minutes, or until the chicken is tender and cooked through. Remove the chicken pieces from the pan and set aside on a warm plate.
Continue to simmer the lemon sauce over a high heat for a further 5-10 minutes, or until the liquid has reduced to the consistency of thin syrup.
Strain the sauce through a fine sieve. Season the sieved liquid, to taste, with salt and freshly ground black pepper and set aside.
Heat the remaining lactose free butter in a frying pan over a medium heat. When the butter is foaming, add the spinach and fry, stirring regularly, for 3-4 minutes, or until just wilted.
To serve, place a portion of the wilted spinach into the centre of each plate. Place two pieces of chicken on top of each portion of spinach. Spoon over the lemon sauce and serve immediately.