Preheat the oven to 350°F (175°C) and place a rack in the middle position. Line a baking sheet with parchment paper.

Toast the walnuts: Spread the 4 cups walnuts in a single layer on the prepared baking sheet and bake 7 to 10 minutes, until fragrant. Leave the oven on for baking the baklava. Let the walnuts cool until just warm to the touch.

Make the filling: In a food processor, combine the toasted walnuts, ½ cup sugar, 1 tablespoon ground cinnamon, and ½ teaspoon ground nutmeg. Pulse/process until the mixture resembles coarse sand and small pebbles. Transfer the mixture to a medium bowl and set aside.

Make the syrup: In a small saucepan, combine 1 cup sugar, ½ cup water, ½ cup honey, 1 tablespoon fresh lemon juice, a big pinch salt, and, if using, 1 cinnamon stick and 2 strips orange rind. Bring to a boil over medium heat, then reduce the heat to low and simmer uncovered for 5 minutes. Remove and discard the cinnamon stick and orange rind (if used), or strain the syrup, then let the syrup cool until ready to use.

Prepare to assemble: Place a 9 x 13-inch (23 x 33-cm) baking dish, a pastry brush, and the melted butter (1 cup) near a flat, clean work surface. Unroll the thawed phyllo dough and, using a sharp knife, trim each sheet to 8 x 12 inches (20 x 30 cm) to fit the baking dish. Discard trimmed scraps. Keep the phyllo sheets covered with plastic wrap and a kitchen towel while you work to prevent them from drying out.

Butter the pan: Lightly brush the bottom and sides of the baking dish with melted butter.

Begin layering phyllo: Working with two phyllo sheets at a time, place 2 sheets in the pan, then drizzle 2 to 3 teaspoons melted butter over them. Continue adding 2 sheets and drizzling 2 to 3 teaspoons butter after every 2 sheets until you have placed 14 sheets total on the bottom (i.e., seven times of 2-sheet units).

Add the first layer of filling: Spread one-third of the walnut filling (about 1¼ cups) evenly over the phyllo layers.

Middle layers and filling: Add 6 more phyllo sheets, continuing to drizzle 2 to 3 teaspoons melted butter after every 2 sheets. After those 6 sheets, spread the second third of the filling evenly. Add another 6 phyllo sheets (drizzling butter after every 2), then spread the final third of the filling.

Top layers: Finish by adding 14 more phyllo sheets, drizzling 2 to 3 teaspoons melted butter after every 2 sheets. Do not brush butter on the very top final sheet.

Score the baklava: Using a very sharp knife, cut all the way through the layers to score the baklava into your desired shapes—24 squares or diamonds are common. If any melted butter remains, spoon or brush it into the cut lines.

Bake: Bake the baklava for 40 to 45 minutes, until the top is golden brown and you hear it bubbling.

Add the syrup: Let the baklava cool 5 minutes in the pan, then spoon or slowly drizzle the cooled syrup into every cut and evenly over the top so it can soak into the layers.

Cool and serve: Allow the baklava to cool completely and absorb the syrup before serving—at least a few hours or overnight for best texture.