Heat the oven to 190ºC/375ºF.
Place the chicken in a large roasting pan.
Insert the whole lemon inside the chicken. Please DO NOT open or cut the lemon, otherwise it won't work.
Season with sea salt and drizzle with olive oil.
Put it in the oven and roast for 40 minutes. Open the oven and make little cuts in the chicken with a sharp knife but be careful not to perforate the lemon. Drizzle the sauce into the chicken with a spoon.
Roast for another 40 minutes. There is no need to turn the chicken.
While the chicken is roasting, boil the veggies in a large pan with a pinch of salt.
When the vegetables are still firm but soft enough to mash, drain and mash them with a potato masher leaving some bits of vegetables for crunchiness (avoid using a ricer, as it will purée the vegetables and it won't be a stoemp).
With a wooden spoon, well incorporate the milk and egg and season with salt, pepper and nutmeg, to taste.
I suggest serving the stoemp in the form of duchess potatoes. In order to do so, force the purée from a piping bag in the desired shape and bake in the oven along with the chicken at 190ºC/375ºF for 40 minutes or until golden.