Preheat the oven to 180°C fan.
Add the onion and mushrooms to a large roasting dish with the olive oil. Season with salt and pepper, toss together and roast for 30 minutes.
Stir in the butter beans, crème fraîche, Marsala wine, chicken stock, bay leaf and herbs.
Nestle the chicken thighs on top and season with a little more salt and pepper.
Bake for 35 to 45 minutes, until the chicken is cooked through and the skin is golden.
Remove the bay leaf and herb stalks, stir through the spinach until wilted, then finish with chopped parsley and chives.
