Preheat the oven to 350˚F.
In a large skillet, heat a drizzle of oil over medium-high heat and cook the onion, carrot and celery for 3-4 minutes, until soft.
Add the garlic and thyme and cook for another minute.
Add the chicken, season with salt and pepper and cook for a few more minutes, until opaque.
Sprinkle with flour, add the stock, cream and peas and bring to a simmer, stirring until the sauce bubbles and thickens.
Pour into a baking dish or pie plate, or into individual baking dishes.
Scrunch each piece of phyllo and place on the surface, and brush with butter.
Bake for 15-20 minutes, or until the pastry is golden and the filling is heated through.