CHICKEN BAKED WITH CIDER AND MUSTARD

2 tbsp olive oil • 4 British chicken thighs • 450g chestnut mushrooms, quartered • 1 large onion, finely sliced • 400g charlotte potatoes, cut into slices • 250ml Aspall Premier Cru Cyder • 2 tbsp Dijon mustard • 300g Tenderstem broccoli, trimmed • 2 tbsp single cream • handful flat leaf parsley, leaves roughly chopped