In a food processor, pulse mushrooms and chickpeas until finely chopped.
In a sauté pan or wide saucepan, melt butter over medium-high heat; fry mushroom mixture, celery, leek and carrot, stirring often, until vegetables are softened, about 5 minutes. Stir in potato, broth, allspice, thyme, salt, cloves and pepper; cook until mixture is thickened, about 3 minutes. Transfer to a bowl; let cool for 10 minutes.
Stir in cheese. In a small bowl, beat eggs; stir all but 1 tbsp (15 mL) into mushroom mixture. Beat 1 tsp (5 mL) water into remaining egg to make egg wash. Spoon filling into one of the pie shells; lightly brush edge with some of the egg wash. Invert remaining pie shell over filling; remove from its foil liner. Pinch shells together; flute or use tines of a fork to press and seal edges.
Brush top of pie with remaining egg wash. Cut 3 steam vents in top. Bake in bottom third of 400°F (200°C) oven until deep golden, about 45 minutes. Let stand for 10 minutes; cut into wedges.
Tip: Serve warm or at room temperature with chili sauce or your favourite chutney.