Step 1 Preheat oven to 200°C. Spray a large deep non-stick frying pan with olive oil; set over high heat. Add diced chicken and cook in 2 batches for 4–5 minutes, or until golden. Remove chicken from pan and set aside.
Step 2 Return frying pan to medium heat; spray with olive oil again. Add leek to pan; cook, stirring occasionally, for 5 minutes, or until soft. Add mushrooms and garlic to pan; cook for 3 more minutes, or until browned.
Step 3 Stir flour into vegie mixture and cook for 30 seconds, then gradually stir in chicken stock and cooking cream; bring to the boil. Transfer reserved chicken to pan with baby peas; simmer for 2 more minutes, or until sauce thickens. Stir mustard through chicken mixture.
Step 4 Cut 4 puff-pastry rounds large enough to cover the tops of 4 x 375ml-capacity ovenproof ramekins. Divide chicken mixture among ramekins, topping each with a 10.5cm pastry round and pressing edges to seal. Bake for 15–20 minutes, or until pastry is puffed and golden.
Step 5 Serve chicken and mushroom pies with steamed broccolini.