Combine oil and lemon juice.
Toss with chicken breasts.
Season with salt and pepper.
Preheat broiler or use grill pan.
Broil breasts 5 minutes per side or until still slightly pink in center.
Cool.
Cut into 1 1/2-inch pieces; reserve.
Preheat oven to 375°F Place garlic in cold water, bring to boil and boil for 5 minutes.
Drain and peel garlic. Reserve.
Cut dark green leaves from leeks and discard.
Wash leeks well and slice into 1/2-inch sections.
Heat butter in skillet on medium heat.
Add leeks and garlic and sauté for about 3 minutes or until leeks soften.
Stir in flour and cook until pale gold, about 4 minutes, adding more butter if needed.
Stir in chicken stock, tarragon, lemon juice and lemon zest.
Bring to boil.
Add cream, reduce heat and simmer for 5 minutes or until thick and glossy.
Season well with salt and pepper.
Stir in green peas and chicken.
Place in a 6-cup baking dish or individual baking dishes.
Roll out pastry 1/4-inch thick to cover top of dish.
Cut a 1-inch strip from edges of pastry.
Brush edges of dish with water and lay strip around edges.
Brush with egg.
Lay remaining pastry on top, sealing edges.
Cut a steam hole, decorate with any extra pastry cut in shapes, if desired.
Brush with egg.
Bake pie on middle shelf of oven for 30 to 35 minutes or until pastry is golden and mixture bubbles.
