Step 1 Preheat oven to 190°C. Make pastry: Combine flour with baking powder in a food processor on low speed. Add olive oil slowly until well combined. Add 60ml cold water, a little at a time, until mixture clumps together. Transfer dough to a floured work surface. Flour hands and form dough into a ball; knead gently until dough is just smooth, then flatten slightly into a disc. Wrap disc in cling film; chill in fridge for 10–15 minutes.
Step 2 Meanwhile, make filling: Heat olive oil in a large non-stick frying pan set over medium–high heat. Add leeks; sauté for 2 minutes. Toss chicken with plain flour and season with black pepper; add to pan and cook for 3–4 minutes, or until browned all over. Add stock and sour cream, stirring continuously until sauce reduces by half. Add mushrooms to pan and remove from heat. Leave pan to cool slightly.
Step 3 Transfer chilled pastry disc to a clean work surface and roll out into a large round to cover pie.
Step 4 Spoon filling into a 22.5cm pie dish and cover with pastry round, trimming to fit. Score pastry with a sharp knife, making 2 or 3 small slits in the centre (to let steam escape). Brush pastry with milk, transfer dish to oven and bake for 50 minutes, or until golden.
Step 5 Remove pie from oven, leave to cool for 5 minutes and serve with steamed green beans.