Roll out the pie crust on parchment paper, then place it in a deep tart pan. Prick the bottom with a fork and refrigerate. Place the black mushrooms in a bowl and cover with hot water. Peel and chop the onion. Cut the chicken into cubes. In a bowl, mix the chicken, onion, and soy sauce. Refrigerate for 30 minutes. Separate the fennel fronds and slice them thinly. Grate the Parmesan cheese.

Heat the olive oil in a skillet. Add the chicken and brown well. Drain and chop the black mushrooms, then add the fennel and mushrooms to the chicken. Cook over low heat, stirring often. In a saucepan, melt the butter, then sprinkle in the flour and mix. Gradually add the milk, whisking constantly, and cook over low heat until thickened. Season with black pepper and add the Parmesan cheese.