1. Heat oven to 425°F.
2. FILLING: Cook carrots, celery and onion in oil in large skillet over medium-high heat, until tender-crisp. Add thyme, salt and pepper to taste.
3. Sprinkle with flour and mix so that all vegetables are coated. Cook about 3-5 minutes, stirring occasionally.
4. Carefully stir in hot chicken stock, reduce heat to medium and continue cooking until sauce has thickened to desired consistency.
5. Add peas and chicken and stir. Pour into 2L casserole dish or 4 individual ovenproof bowls.
6. CANOLA OIL PASTRY: Meanwhile, combine flour, and salt in large bowl. In separate bowl, combine oil and water. Add to flour, mixing all at once, using a fork to combine.
7. Dough will look mealy . Line rolling surface with waxed or parchment paper.
8. Drop dough onto prepared surface and knead until dough comes together. Dough will be sticky and difficult to work with. Place another piece of waxed or parchment paper on top of dough and roll with rolling pin to 1/4 thickness.
9. Brush beaten egg around edges of casserole dish. Carefully place pastry on top, pushing down on edges so they stick to casserole. Trim pastry and save for another use.
10. Cut slits in top for steam to escape and brush with remaining beaten egg.
11. Bake in preheated oven for 20-25 minutes, until pastry is golden brown. Serve immediately. Filling will be very hot!