Preheat the oven to 200C/180C Fan/Gas 6.
Place the onions and carrots in a deep-sided roasting tin, then sit the chicken on top. Season the chicken all over with salt and freshly ground black pepper, then drizzle over the rapeseed oil. Pour 75ml/2½fl oz water into the tray.
Roast for 1-1¼ hours, or until the chicken is cooked through. (The chicken is cooked if the juices run clear when a skewer is inserted into the thickest part of the thigh meat. If the chicken is not cooked through, return to the oven for a further 10 minutes and check again.)
Remove the chicken from the oven and set aside, covered, to rest for 10 minutes.
Meanwhile, for the fricassée, bring half of the butter and 200ml/7fl oz water to the boil in a saucepan. Reduce the heat until the water is simmering, then add the asparagus, peas, shallot and marjoram. Simmer for 3-4 minutes, or until the vegetables are all tender.
Drain the vegetables, if necessary, and return to the pan. Stir in the remaining butter and season, to taste, with salt and freshly ground black pepper.
To serve, carve the meat and place it on serving plates. Spoon the fricassée alongside.