Salt and pepper the chicken thighs.
Heat the olive oil over medium heat.
Add the thighs to the frying pan and cook until golden on each side; about 5 - 7 minutes per side.
Remove the thighs and set on a plate.
In the same frying pan with the chicken fat, add the leek, stir and cook until soft over low heat. This will take about 5 minutes.
Add the garlic and thyme. Stir and cook for 1 minute.
Deglaze the pan with brandy or the Calvados.
Place the chicken back into the dutch oven, along with any juices that collected on the plate.
Pour the cider over the chicken, stir and then cook over low heat for another 10 minutes.
Stir in the mushrooms, and cook for another 10 - 15 minutes. Until the chicken registers 140 on a meat thermometer. Stirring occasionally Stir in the heavy cream, and season to taste with salt and pepper. Then cook for another 5 minutes until thickened.
Stir in the parsley.