Preheat oven to 425°F. Generously spray 6 cups of a standard muffin pan. Whisk egg yolk and water in a small bowl and set aside.
In a large sauté pan over high heat, melt butter with oil and add mushrooms, cauliflower, thyme and garlic. Cook for 5 minutes. Reduce heat and stir in flour to coat vegetables. Stir in cream. Take off the heat and stir in cheeses, nutmeg and lemon juice. Season with salt and pepper. Set aside.
Remove pastry shells from foil and place onto your flour dusted work surface. Gently knead pastry into a ball. Using a rolling pin flatten pastry to a ½-inch thickness. Using a 4-inch round pastry cutter, cut out 6 rounds. Re-roll pastry if needed to make cut outs.
Press pastry cut outs into each cup of the muffin pan. Create an edge, roughly ½ of an inch, pressing the pastry up over the lip of the cup to ensure a seal for the top pastry. Re-roll pastry as needed and cut out 6; 3-inch, ¼ inch thick rounds for the tops.
Fill pastry with mixture to the brim and top with smaller pastry cut outs. Press down or pinch pastry edges to seal. Brush with egg wash and pierce pastry creating air holes. Sprinkle with salt and bake for 20 -25 minutes, until pastry is deep golden brown. Allow pastry to cool for 10-15 minutes before removing from the pan. Serve with a side salad.