1-First Step: Prepare Oven and Ingredients Preheat your oven to 200°C (400°F). Cut chicken into bite-sized pieces, wash and slice mushrooms, dice onion, and mince garlic. Prepare puff pastry by thawing it in the fridge but keeping it cold.

2-Second Step: Sauté Aromatics and Mushrooms Melt 1 tablespoon butter in a large skillet over low heat. Add diced onion and cook gently for about 5 minutes until fragrant and translucent. Increase heat to medium and stir in mushrooms; cook until golden and tender (5-7 minutes). Add minced garlic and cook for 30 seconds more. Season with salt and pepper, then transfer mixture to a bowl to cool temporarily.

3-Third Step: Cook Chicken and Build Filling In the same skillet, melt the remaining butter over medium-high heat. Brown chicken pieces on all sides but do not cook through fully. Add fresh tarragon or thyme and stir. Sprinkle flour evenly over chicken and mix well to coat. Gradually pour in chicken stock, stirring constantly to avoid lumps, and cook until sauce thickens.

4-Fourth Step: Add Cream and Mustard Lower heat to medium-low and stir in heavy cream and mustard. Simmer gently for about 5 minutes until the filling is rich and creamy. Adjust seasoning as needed. If using frozen peas, stir them in now. Remove from heat and allow filling to cool for about 30 minutes to prevent soggy pastry.

5-Fifth Step: Assemble the Pie Transfer the cooled filling to a pie dish or leave in an oven-safe skillet. Roll out the puff pastry to about ¼ inch thickness and cover the filling, sealing edges well. Brush the pastry with beaten egg and cut slits or an X on top for steam to vent.

6-Final Step: Bake and Rest Bake pie in the middle rack of the preheated oven for 25 to 30 minutes or until the pastry is golden brown and filling bubbles. Let rest for 20 minutes before slicing and serving for easier portions and to enhance flavors.