Brown the chicken: Preheat oven to 180°C fan bake. Heat a medium oven-safe pot with 20g of butter over medium heat. Slice 2 chicken thighs into pieces. Add half the chicken and par-cook until browned. Remove and repeat with the remaining chicken, seasoning well with 1 tsp salt & pepper. Once brown, remove & set aside.
Cook the veggies: Dice 1 small onion, 1/2 leek and 200g mushrooms into small chunks. In the empty chicken pan, add another 20g of butter, then add chopped vegetables. Season with 1/2 tsp salt & pepper and cook for 2-3 minutes.
Make the sauce: Return the chicken to the pan of vegetables. Stir in 100ml cream, 1.5 tsp beef stock powder, 1 cup water, 1 tbsp corn flour, and 1 tbsp Dijon mustard. Mix well and simmer for 2 minutes, then remove from heat.
Assemble: If your pan isn’t oven-proof, transfer the filling to a 30cm baking dish. Brush each filo sheet with oil, lightly scrunch, and place on top. Repeat with enough filo sheets to ensure full coverage.
Bake & serve: Bake for 25 minutes or until the pastry is golden brown. Serve warm.