Heat the oil in a large frying pan over medium heat. Cook the onion, celery and garlic for 5 mins or until softened but not yet brown. Increase heat to high. Add the chicken and cook, stirring, for 3-5 mins or until brown. Add the cauliflower and broccoli. Cook for 2-3 mins or until vegetables soften.
Add the flour and cook for 1 min. Add the cream and mustard and bring to the boil. Reduce heat to medium and simmer for 5 mins or until thickened slightly. Spoon the chicken mixture into four 1¼ cup (310ml) ramekins.
Preheat oven to 220C. Use a 10cm round pastry cutter to cut four rounds from the pastry. Place each round over the chicken mixture, pressing the edges to seal. Make a small slash in the centre of each pastry round. Brush with a little milk. Bake for 20 mins or until puffed and golden. Serve the pot pies with the mixed salad leaves.
