Preheat the oven to 450°F.

Place the chicken in a round 3-quart ovenproof tureen. Core the tomato, cut into eighths, and remove the seeds. In a medium saucepan, bring the broth to a boil and reduce to a simmer. Add the carrots, turnips, onions, string beans, and peas and cook for 3 minutes. Strain, reserving the broth. Cool the vegetables under cold running water, and drain. Add the vegetables and tomato (and frozen peas, if using) to the tureen. Dot with butter and sprinkle with the optional truffle. Season with salt and pepper.

Roll out the puff pastry on a lightly floured cold surface, to about a 3/16-inch thickness. Cut out a circle of pastry about 2 inches larger than the diameter of the tureen. Brush the circle lightly with the beaten egg. Fit it over the tureen, brushed-side-down, pressing the overhanging pastry against the sides of the tureen. Brush the top side lightly with additional egg. Bake for 10 minutes. Carefully cover the pastry loosely with heavy aluminum foil and continue baking for 35 minutes until puffed and golden brown. Turn the oven off. Leave the tureen in the closed oven for 20 minutes, then remove.

Bring the tureen to the table and carefully cut off the crust, just inside the rim. Set the crust aside. Remove the bird and carve it on a separate platter. Serve in warm soup plates with the vegetables, juices, and a bit of the crust.