Preheat oven to 425°F. (If making ahead skip this step.)

In a stock pot or Dutch oven, melt the butter over medium-high heat.

Add onion, potatoes, carrots and mushrooms and cook, stirring occasionally, until tender, about 10-12 minutes.

Add cognac, and combine.

Stir in flour and cook for about 1 minute until combined well.

Add chicken stock and milk. Combine well and bring to a simmer for about 2-minutes, stirring until mixture thickens.

Add chicken, peas, parsley, thyme, sage, salt and pepper, and combine well.

Transfer mixture to preferred serving vessel. Or if preparing ahead, let cool and refrigerate to re-heat later.

When ready to bake, whisk together the egg yolk and 1 teaspoon of water.

Cut thawed puff pasty 1″ larger in diameter than the chosen serving dish/vessel, and cut out a 1/2-1″ hole in center for venting.

Center the cut puff pastry over dish and crimp down around edge of dish to seal.

Brush top of pastry with egg wash.

Bake in a 425°F oven, placing your dish on a baking sheet, for 10 minutes, then cover lightly with aluminum foil and continue baking for another 20-25 minutes.