Maple Turkey, Sweet Potato, and Foie Gras Sheet Pie - Maple from Canada

Ingredients

• 2 cups sweet potatoes, cubed
• 1 tbsp butter
• 1 onion, chopped
• 3 tbsp maple syrup (preferably amber syrup for its rich flavor)
• 1/4 cup foie gras mousse or paté, plain or with truffles
• 2 cups turkey, cooked and shredded
• 1 lb tore-bought pie or puff pastry dough, rolled out
• 1 egg, beaten
• 1 - 2 tsp maple sugar
• 1/4 tsp fleur de sel
• Salt and freshly-ground pepper