Heat 2 tablespoons extra virgin olive oil in a large skillet.Add 1 medium leek (cut in half and sliced), 1 large onion, 1 large carrot, 1 rib celery, and 4 cloves garlic (all chopped), and cook for 5 - 10 minutes, stirring often.Add 2 tablespoons tomato paste, 1 teaspoon rosemary, 1 teaspoon sage, ½ teaspoon thyme, and cook for 2 more minutes.

Meanwhile, rinse and coarsely chop 1 pound mushrooms and add them to the pan.Add 1½ cans (15 oz each) lentils (drained and rinsed), 1 cup walnuts (chopped), 4 tablespoons rolled oats, then season with 2 tablespoons soy sauce, 1½ teaspoons salt, and ⅛ teaspoon black pepper. Cook 15 minutes, stirring occasionally. Taste, adjust, and let cool down to room temperature.

Preheat oven to 350°F or 180°C. Line a baking tray with parchment paper. Arrange a 10 to 14-ounce puff pastry sheet on baking tray to create a rectangle of at least 12 x 8 inches (30 x 20 cm).Make diagonal cuts on the sides of the puff pastry sheet (about 1 inch / 2.5 cm wide). Arrange the mushroom filling in the center, leaving some space at the margins, and compact it with your hands or a spoon.

Fold in the strips to create a cross-hatch pattern and wrap the filling.Brush with heavy cream, milk, or their non-dairy substitutes.

Bake at 350°F or 180°C for about 30 minutes or until golden brown.Let it cool down for 5 minutes, then cut it with a serrated knife. We recommend serving it with mushroom gravy.