- 4 tbs unsalted butter, divided
- 1 tbs olive oil
- 2 small leeks, sliced, washed, and drained
- 225 g (3 cups) white mushrooms, cleaned and sliced
- 3 tbs plain flour
- 600 ml whole milk
- 200 ml chicken stock, plus extra as needed
- 4 medium skinless chicken breasts, diced
- 3 large carrots, peeled and sliced
- 225 g (1 1/2 cups) frozen peas, thawed
- 3 large Russet or Desiree potatoes
- 3 tbs butter, melted
- Freshly ground black pepper
Melt 1 tablespoon unsalted butter with the olive oil in a large saucepan set over a medium heat until hot.
Add the leeks, mushrooms, and a pinch of salt, sweating until softened, about 10 minutes. Remove from the saucepan to a plate. Wipe the saucepan clean with kitchen paper.
Set the saucepan over a medium heat and add the remaining unsalted butter. Once melted, whisk in the flour, cooking for 2 minutes until blonde and bubbling.
Gradually whisk in the milk and chicken stock in a slow, steady stream until incorporated and smooth.
Bring to a simmer and cook for 8-10 minutes until thickened to a gravy consistency; thin out with more chicken stock if needed.
Fold in the diced chicken, carrots, peas, sweated leeks and mushrooms, and some salt and pepper to taste. Spoon into a rectangular baking dish. Preheat the oven to 190C.
Peel the potatoes and cut into thick matchsticks. Place in a colander and rinse under running water before spreading out on kitchen paper to drain.
Arrange the potato on top of the filling and brush with the melted butter. Season with plenty of salt and pepper.
Bake until the potato is golden-brown on top, about 30-40 minutes. Remove from the oven and let stand for at least 5 minutes before serving.