Preheat the oven to 425°F (218°C).
Heat the oil over medium-high heat in a medium saute pan until hot.
Season the chicken with pepper and add to the pan. Cook, stirring sparingly, until golden brown on one side, 5 minutes.
Using a slotted spoon, transfer to a bowl, leaving oil and drippings in the pan.
Reduce heat to medium and add the leeks, carrots, celery and rosemary to the pan and saute until tender, 5 minutes.
Meanwhile, put the flour in a small bowl and gradually whisk ½ cup (118ml) broth into the bowl until smooth.
Add the flour-broth mixture and the remaining broth to the saute pan with the vegetables, and stir constantly until the mixture is thickened and bubbly, 3 minutes.
Return the chicken and any accumulated juices to the pan.
Stir in the peas, lemon zest, salt and pepper.
Reduce heat to low and simmer gently for 5 minutes, or until the chicken is cooked through and registers 165 degrees F.
While the filling simmers, make the topping. Mist a sheet of phyllo dough with olive oil spray and crumple it to make a small mound.
Put it on a baking sheet and repeat with the remaining phyllo sheets.
Bake until the dough mounds are golden brown, about 5 minutes. Watch carefully to make sure they don’t burn.
Put the phyllo mounds on top of the filling in the saute pan.
Bring to the table and serve.
