Place the chicken in a large pot. Add half of the onion and the bay leaf and fill the pot until the chicken is covered with the water. Place over medium heat and cook until the chicken is cooked through, about 20 minutes.

Remove the chicken from the water and allow to cool slightly, then shred or dice. Discard the onion and the bay leaf. Place the chicken in a large bowl.

Preheat the oven to 400ºF. Spray 8 muffin tins very generously with nonstick cooking spray.

In a large skillet, heat the olive oil over medium-high heat. Add the mushrooms and cook until softened. Dice the remaining onion and add to the skillet along with the celery. Season the vegetables to taste with salt and pepper, and cook until softened. Add in the frozen peas and carrots and cook for 2 more minutes. Remove the vegetables from the skillet and add to the bowl with the chicken.

Return the skillet the the heat and add the butter. Whisk in the flour and allow it to cook for a minute or two. Slowly whisk in the chicken broth and cook until it is thickened, about 1-2 minutes. Stir in the cream. Add the mustard, thyme, and dill and season to taste with salt and pepper. Pour the gravy over the chicken and vegetables and stir to combine.

Lightly flour a counter and roll the puff pastry out to a rectangle about 16”x10”. Cut the dough into 8 rectangles. Place each piece of puff pastry in one of the prepared muffin cups and divide the mixture evenly between the 8 cups. They will be very full. Fold the points of the dough over the filling so that they meet in the middle.

Lightly beat the egg with a splash of water. Brush the egg mixture over the pastry.

Bake in the preheated oven until they are golden brown, 15-18 minutes. Let the mini pot pies stand for a few minutes before removing from the muffin tin to serve.