Peel and dice the celeriac into large cubes. Put it in a pan and cover with water. Cook for around 20 minutes. When the pieces are soft, blend them in a food processor with the salt, pepper and nutmeg. Add 25g butter to the purée. Leave to one side.
Using a mandolin, thinly slice the raw beetroot. Leave to one side.
Peel the onions.
Put the pearl onions in a frying pan with 30g butter, making sure not to overlay them. Add the sugar and salt and fill the pan halfway with water.
Cover the frying pan with a sheet of greaseproof paper with a hole in the centre. After around 10 minutes of cooking, remove the greaseproof paper and continue to let it evaporate.
Continue cooking, stirring from time to time to properly coat the onions in the caramel that is forming. Remove from heat when the onions are lightly browned and glossy. Leave to one side.
Crush the pepper roughly in a mortar. Pour the crushed pepper into a bowl. Place each tournedos in the pepper, pressing down firmly on each side so that the pepper sticks to the surface.
Cook the meat in 10g of butter on a high heat in a non-stick pan for 3 minutes on each side. Keep the meat warm. Add the Isigny Sainte-Mère Cream & Foie Gras to the pan, bring to a low boil, then turn off the heat.
To serve, place two tablespoons of the purée, then the tournedos, a few caramelised onions, two tablespoons of Isigny Sainte-Mère Cream & Foie Gras, a few slices of raw beetroot and some chervil leaves on a plate.