In a casserole, pour broth and add chicken thighs. Bring to a boil then simmer 10 minutes at medium heat.
Add sliced leeks and continue cooking for 5 minutes. Filter broth. Set chicken thighs and leeks in a large plate. Cool down then dice chicken thighs.
In a casserole, heat butter and brown onion for 1 to 2 minutes. Add mushrooms and cook 3 minutes. Spread flour and stir. Pour filtered broth and cream. Bring to a boil while stirring.
Add chicken dice, leeks, cheese and parsley. Season and stir. Remove from heat and cool down.
Preheat oven to 205ºC (400ºF). Slice puff pastry in two equal parts. Over a floured work surface, set down half the pastry into a square of 25 cm (10 inches) and lay down in a gratin mold of 25 cm (10 inches). Pour the mixture in the mold then brush pastry sides with whisked egg yolk.
Set down the other half in a square of 23 cm (9 inches) and incise its center at few places. Cover with second pastry and sal sides.
Brush top of the crust with egg yolk. Oven bake for 30 to 35 minutes, until pastry is well browned.
Did you know that...
Each package of 250 g of sliced leeks contains 4 cups of sliced leeks, washed and ready to cook.