Heat half the oil in a large saucepan or deep frying pan. Cook the chicken in batches until just golden. Remove from the pan and set aside.
Heat the remaining oil in the same pan. Add the leek, carrot, capsicum, shallots and mushrooms and cook for about 5 minutes, stirring regularly, until the leek and mushrooms become soft. Add the chicken back into the pan.
Add the flour and cook over medium-high heat for 1 minute. Dissolve the chicken stock powder into the water and add to the pan, along with the rosemary and thyme. Bring to the boil and simmer uncovered for about 15 minutes or until the liquid begins to thicken.
Add the cream and wholegrain mustard and stir to combine.
Spoon the mixture into four individual ramekins. Divide the puff pastry sheet into quarters and top each ramekin with pastry. Brush lightly with milk and bake in a moderate oven for about 15 minutes or until pastry is just browned. Serve with a fresh green leafy salad.