Heat the oven to 200C/180C fan/gas 6. Remove any string from the chicken and sit in a large roasting tin or a baking dish, with plenty of space around it.
Mash 2 tbsp of the mascarpone with the lemon zest, 1 tbsp of the tarragon and some seasoning. Slip your hand beneath the chicken skin to pull it away from the meat, then spread the mixture beneath the skin in a thin layer. Spoon another 3 tbsp mascarpone into the cavity of the chicken, to melt in with the roasting juices and enrich the sauce later on. Rub 2 tbsp olive oil into the skin, season well with sea salt, then loosely tie the legs together with butcher’s string. Roast for 20 mins.
Arrange the potatoes and the garlic around the chicken, drizzle over another 1 tbsp oil and cook for another 30 mins.
Toss the radishes and whole spring onions into the dish, in and around the potatoes, coating everything in the fat, then roast for another 25 mins. The potatoes and radishes will be golden and tender, and the chicken should be cooked through. Remove the chicken from the tin, cover loosely with foil and leave to rest.
Pour off or spoon away the excess oil from the tin. Stir the remaining mascarpone (about 150g) with the stock in a jug until lump-free, then pour into the tin and bubble on the hob for few minutes, stirring to coat the potatoes and veg. Squeeze over some lemon juice and season.
Stir in the peas, spring greens and most of the remaining tarragon, and bubble for a few more minutes until bright green. Sit the chicken back in the middle of the tin to serve and scatter over the reserved tarragon.