Heat oil in a large, non-stick skillet over medium high heat. Add the beef sirloin cubes and cook, stirring occasionally until well browned. Season with salt and pepper and remove from skillet. Meanwhile preheat oven to 250 degrees.
To the same skillet add the butter. When it is melted add the mushrooms and cook until browned, stirring occasionally. Add the carrots and onions and cook for about 5 minutes, stirring occasionally.
Add the cooked beef, mushrooms, carrots and onions to an oven-safe pot. Add the wine, thyme and bay leaf. Stir to combine.
Cover the pot with a lid. Place in the preheated oven on the center rack. Roast for 3 hours.
In a small bowl, stir together the cornstarch and water.
Remove the stew from the oven and stir in the cornstarch mixture. Allow the stew to cool completely. The stew can be refrigerated in an air-tight container for up to two days.
When ready to make the pot pies, gently reheat the beef stew in the pot on top of the stove.
While the stew is reheating, preheat the oven to 400 degrees.
Cut four rounds out of the puff pastry to fit the tops of four serving bowls. Cut hearts out of the centers of each puff pastry round.
Place the puff pastry rounds on a baking sheet lined with parchment paper or a silicone baking mat. Bake the rounds for 20 minutes.
Spoon the beef burgundy stew into four bowls. Top each bowl with a puff pastry round. Serve immediately.