Prepare the Pastry: Bake the puff pastry shells according to the package directions until they are golden brown and puffed.
Set them aside while you prepare the filling.
Sauté the Mushrooms: In a medium skillet over medium heat, melt the butter.
Add the chopped mushrooms in a single layer and cook without stirring for 5 minutes. This allows them to develop a deep, brown sear, which is key for flavor.
Add Aromatics: Stir the mushrooms, then add the salt and chopped rosemary.
Continue to cook for another 3 minutes, stirring occasionally, until the mushrooms are tender and fragrant.
Create the Ragout: Pour in the vegetable stock and bring it to a simmer.
Whisk in the cornstarch and water mixture (slurry), and stir continuously until the sauce thickens and bubbles, beautifully coating the mushrooms.
Finish and Season: Remove the skillet from the heat.
Adjust the seasoning with a generous amount of freshly ground black pepper and a small squeeze of lemon juice to brighten the flavors.
Taste and add more salt if necessary.
Assemble and Serve: Carefully spoon the hot mushroom ragout into each of the baked puff pastry shells.
Serve immediately while piping hot for the best experience.
Don't overcrowd the pan when cooking the mushrooms.
Cook them in batches if necessary. Giving them space is the key to getting a deep, savory brown sear instead of just steaming them.
