Preheat oven to 425°F (220°C) and line baking sheet with parchment paper.
On clean, lightly floured surface, place 1 sheet puff pastry and unfold. Using rolling pin, lightly roll to close any cracks in pastry.
Spoon 2 heaping tablespoons jam in center of pastry, then place 1 8-ounce wheel brie directly on jam.
Fold pastry up over brie from all sides, forming neat package. Using hands, gently press edges of pastry against brie to neaten sides and pinch seams together to seal shut.
If desired, trim excess pastry and cut decorative shapes to place on top of wrapped brie. Create decorations by cutting shapes with paring knife or punching out with cookie cutter.
Place wrapped brie bundle seam-side down on prepared baking sheet.
Using pastry brush, brush 1 beaten egg over tops and sides of brie bundle. If decorating with pastry scraps, brush those with beaten egg and set on top of wrapped brie, pressing lightly to adhere.
Transfer baking sheet with brie to preheated 425°F (220°C) oven and bake until pastry is deeply golden brown, about 20 minutes.
Remove from oven and let rest 5 to 10 minutes before serving.
Transfer baked brie to serving platter and garnish with fresh thyme leaves if using. Serve with crackers, sliced baguette, or fresh fruit. Brie en croute is best served warm or at room temperature.