Place the chicken breasts in a pot with 1 quartered onion and 1 bay leaf. Fill the pan with just enough water to come to the top of the chicken breasts. Bring the mixture to a low boil, reduce the heat to a simmer and poach the chicken 12 to 15 minutes. Remove the chicken and dice or shred with forks. Reserve the poaching liquid.
In the same pan, heat 2 tablespoons extra-virgin olive oil, a couple of turns of the pan over medium-high heat, add the mushrooms and lightly brown. When the mushrooms are just tender, add 2 chopped medium onions, potatoes, parsnips, celery, sage, salt, pepper and remaining bay leaf. Cover the pot and sweat the vegetables 10 minutes, stirring occasionally. Remove the bay leaf. Melt the butter in the pot, add the flour and stir to make a light roux. Deglaze the pan with a splash of white wine and add one to 2 cups of reserved poaching liquid. Whisk in the mustard and 2 cups chicken stock. Stir in the cream. Thicken to a light gravy consistency. Adjust the salt and pepper and add in the chicken.
Reduce the heat to low and bake off the pastry tops.
For make-ahead meal: Bake off the pastry tops; cool the filling and store. Reheat the filling, covered, over medium heat to a bubble, and then reduce the heat to a simmer.
To prepare the pastry tops: Cut the pastry into shapes which reflect the shape and size of the dishes or bowls you wish to serve the soup in. Beat the egg with a splash of water and arrange the cut pastry on a nonstick baking sheet or on parchment-lined sheet pan. Brush the dough with the egg wash and bake to golden at 425 degrees F, 12 minutes.
Chicken and Mushroom filling can also be served as a thick soup with crusty, warm baguette for dipping.