Preheat the oven to 425°F (220°C).
Spray GENEROUSLY a regular-sized muffin tray with cooking/baking spray or grease it with vegetable oil.
Cut brie into 1 1/2 inch (4cm) wide and 1/2 inch (1 cm) thick slices.
Your puff pastry should be about 1/8-inch (3-4 mm) thick, if it's thicker and square roll it out on a piece of parchment paper into a 9x15 inches (23x38cm) rectangle.
Cut the puff pastry into 3-inch (7-8 cm) squares (3 rows, 5 columns).
Place each square in the center of each muffin cup.
Place a piece of brie in each pastry cup.
Spoon a small amount of cranberry sauce on top, about 1-2 teaspoons (don't add too much or the pastries will be too sweet).
Place the muffin pan in the oven and bake the pastries for 10-12 minutes or until they are puffed and nicely golden. The baking time may vary depending on the oven so keep an eye on them at the end of the baking time to make sure they don't get too dark.
While the pastries are baking, chop the walnuts into smaller pieces, toast them in a dry pan until lightly golden and fragrant. Transfer them immediately to a plate, they can burn quickly in a hot pan.
Take the pastries out of the oven, transfer them from the muffin tin to a cooling rack.
Sprinkle with chopped toasted nuts and thyme leaves.