Heat oven to 180C/160C fan/gas 4. Season the chicken thighs well and then dust all over with the gluten free flour. Heat 1 tbsp of the oil in a paella pan or large, deep frying pan. Fry the thighs until golden brown all over, then transfer to a shallow roasting tin and finish in the oven for 30-40 mins.
Add the remaining oil, and fry very gently until soft, about 10-15 mins. Stir in the rice, paprika, saffron and lemon zest, then add the stock and simmer, stirring occasionally, for about 20 mins on a medium heat until the rice is nearly cooked.
Add the green beans and juice of 1 lemon until the rice is just cooked through, along with the veg. Stir through the herbs and as much of the remaining lemon juice as it needs, along with some seasoning. Tuck the chicken thighs back in and cover for 5 mins to let everything settle before serving.