Line a 12 inch (30 cm) pie plate with pastry dough. Trim and crimp the edges. Set aside in the refrigerator. In a skillet, heat 1 tbsp (15 mL) butter or margarine. Add onion and garlic and saute over medium heat until soft. Add mushrooms and cook until golden. Transfer to a bowl and set aside. Heat remaining butter or margarine, add spinach and saute. Combine with mushroom mixture, then let cool. Preheat oven to 375 F (190 C). In a medium bowl, while beating eggs, add half and half cream, lemon zest and nutmeg. Scatter2 1/2 cups ( 625 mL) cheese on the bottom of tart, followed by the cooled vegetables. Spread evenly. Pour egg mixture over. Sprinkle remaining cheese on top of tart (optional). Bake on bottom rack of oven for 40 minutes or until golden and the filling is set.