This hearty pie is a banging choice for dinner.
Preheat the oven to 200°C. Melt the butter in a large saucepan, add the chicken and fry in batches for 4-5 minutes until golden. Transfer to a plate with a slotted spoon. Add the mushrooms to the pan and fry for 3-4 minutes until golden. Remove the mushrooms from the pan and transfer to the plate with the chicken.
Pour the milk into the saucepan, bring to a simmer then whisk in the gravy granules. Cook for a minute until the sauce starts to thicken then return the chicken and mushrooms to the pan. Remove from the heat, stir in the crème fraîche, mustard, spinach and tarragon and season to taste with salt and pepper. Spoon the chicken mixture into a pie dish.