Preheat the oven to 350 degrees F.
For the hazelnut dukkah: Toast the hazelnuts in the oven for 12 minutes. Let cool. Raise the oven temperature to 400 degrees F.
Meanwhile, toast the coriander and cumin seeds in a saute pan over medium heat, about 1 minute.
Grind the coriander, cumin and hazelnuts in a spice grinder. Mix with the sesame seeds.
For the spinach pie: In a large skillet, saute the garlic and shallots in the butter over medium heat until softened. Add the spinach and cook until wilted. Add the dill, nutmeg and some salt and pepper. Let cool slightly, then stir in the Cheddar and mozzarella.
Lay one sheet of puff pastry in a 9-by-13-inch baking dish. Spread with the spinach stuffing. Lay the other sheet on top, pinch the edges closed, and cut into six pieces. Sprinkle with dukkah and brush the edges with egg wash. Set the baking dish on a baking sheet and bake until the pie is golden brown, about 30 minutes.
Sprinkle with the sumac and serve.