For the roast chicken, preheat oven to 180C/375F/Gas 5.
Push your fingers under the chicken skin to loosen the skin away from the breast. Mix the cream cheese and all the herbs in a medium bowl until well combined. Season with salt and freshly ground black pepper.
Transfer the cream cheese mixture to a piping bag and pipe it under the chicken skin over the breast. Place the halved lemon in the cavity of the chicken.
Place the chicken into a large roasting tin and cook for 1½-2 hours, or until the juices run clear when the bird is pierced in the thigh with a skewer.
For the crème fraîche and herb rösti, place the grated potatoes into a clean tea towel inside a bowl and squeeze out any excess water into the bowl.
Transfer the grated potato to a clean bowl along with the crème fraîche, herbs and egg yolk and season well with salt and freshly ground black pepper. Stir until well combined.
Heat the butter in an ovenproof frying pan over a medium heat and place 2 x 7.5cm/3in chefs' rings into the pan. Spoon the potato mixture into the rings, press down well with a spoon, and cook for 3-4 minutes, or until golden-brown. Remove the ring, turn the rösti over, press the mixture down and cook for 3-4 minutes on the other side.
Transfer the pan to the oven and bake the rösti for 5-10 minutes, or until tender and cooked through.
For the sugar snap peas, runner beans and Chantney carrots, cook the carrots, peas and beans in a pan of boiling water for 2-3 minutes. Drain the vegetables well and refresh in a bowl of iced cold water. Drain again once cool.
Heat a medium frying pan, add the butter and fry the blanched vegetables for two minutes.
To serve, cut the breast and thighs off the chicken and cut the breast into slices.
Heat the the tray used to roast the chicken on the stove-top. Serve the röstis with the chicken and vegetables and spoon the warm chicken juices over.
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James Martin presents, with guest chefs Marcus Wareing and Sabrina Ghayour.