Preheat the oven to 180C/350F/Gas 4.
For the roast chicken, rub the butter and thyme leaves onto the skin of the chicken.
For the boulangère potatoes, mix the potato and onion slices together in a bowl and season, to taste, with salt and freshly ground black pepper. Transfer the mixture to a large buttered ovenproof dish and pour over the stock.
Transfer the dish to a rack in the lower part of the oven. Place the chicken directly onto the shelf above the potatoes so that any fat from the chicken drips onto the potatoes. Roast for 1½-2 hours, or until the potatoes are tender and golden-brown and the chicken is cooked. (The chicken is cooked through when the juices run clear when pierced in the thickest part with a skewer.) Once cooked, removed from the oven and set aside to rest for 10 minutes.
For the salad, whisk shallot, mustard, vinegar and olive oil together in a bowl until well combined. season, to taste, with salt and freshly ground black pepper.
Mix the Little Gem lettuce and salad dressing together in a bowl until well combined.
To serve, carve the chicken into thick slices and place some onto each of four serving plates. Spoon the boulangère potatoes and salad alongside.
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