When the chicken is cooked, remove to a platter, cover loosely in foil and leave to rest for 15 minutes. While the chicken is resting, put the shallot into the tray with the chicken juices, and cook over a medium heat (transfer to a frying pan if your roasting tin doesn’t work on the hob). Fry for 6-8 minutes or until translucent. Stir in the mustard and stock. Simmer until reduced by half, then stir in the crème fraîche and bring to a bubble. Stir in the tarragon and broad beans, season with salt and pepper, and a squeeze of lemon. Cook until the broad beans are tender.