Put chicken thighs in a saucepan, pour 3 liters of boiling water, bring to the boil, add salt and skim off the foam. Clean the mushrooms and separate the stipes from caps. Peel an onion, stick a carnation into it, put it in a saucepan with thighs and mushroom stipes. Cut celery into large pieces and add to the saucepan. Cook with gentle boiling until soft meat, for about 30 minutes. Remove the meat from the bones and cut into small pieces. Put bones back in the broth. Cook the broth under a high heat and evaporate two-thirds. Strain out the broth. Chop the champignon caps into 8-10 pieces. Melt 30 g butter in a deep frying pan and fry the champignons under a high heat, often stirring so that all the liquid is evaporated. Add mushrooms to the meat. Melt the remaining oil in a frying pan. Pour in the flour, stir thoroughly and fry for 3 minutes, stirring. Pour in the broth, stir with a wire whisk and cook for 4 minutes, stirring. Season with salt, pepper and nutmeg, mix with the thigh meat, grated cheese and mushrooms. Pour into ramekins for baking. Bake in a preheated oven to 180 °C until golden brown for about 5 minutes. Serve very hot.