Mushroom, leek and chicken pie

Ingredients

• 400 g cup mushrooms, sliced
• 1 large brown onion, roughly diced
• 2 garlic cloves, minced
• ¼ bunch parsley
• 1 kg chicken thighs, roughly chopped
• 550 ml full cream milk, plus extra for brushing over pastry
• 1 ½ tsp whole-grain mustard
• 1 chicken stock cube
• 2 tbsp fresh tarragon leaves, roughly chopped
• 3 sprigs of fresh thyme leaves
• 80 g plain flour
• 1 large leek, thinly sliced
• 2 knobs of butter
• ½ cup frozen peas
• 200 ml creme fraiche
• 1 sheet frozen puff pastry, defrosted
• cracked black pepper
• olive oil for cooking