How to Make It
Preheat the oven to 350°. Butter a 9-by-13-inch baking dish. Spread the hazelnuts on a rimmed baking sheet and bake until fragrant and the skins blister, about 12 minutes. Transfer the hazelnuts to a kitchen towel to cool completely. Rub the nuts together in the towel to loosen the skins; don't worry if some aren't totally peeled. Transfer the peeled hazelnuts to a food processor, add 1/4 cup of the sugar and pulse until finely chopped.
Set the phyllo dough on a work surface and cut the stack in half crosswise; you should have 2 stacks of 8-by-12-inch rectangles. Cover 1 stack of phyllo sheets with a lightly damp paper towel and plastic wrap. Layer the remaining phyllo in the prepared baking dish, lightly brushing each sheet with melted butter. Spread the finely chopped hazelnuts over the phyllo in an even layer. Cover with the remaining phyllo, lightly brushing each sheet with melted butter and saving the nicest 2 or 3 sheets for the top layer. Brush the top of the baklava evenly with butter and press lightly to compact it.
Using a ruler and a sharp paring knife, cut the baklava lengthwise into 2-inch strips, then cut the strips on the diagonal to form diamonds.
Bake the baklava until golden, about 25 minutes. Turn the oven down to 300° and continue baking until the baklava is browned, about 30 minutes longer. Transfer the baking dish to a rack. Run the knife through the cuts to make sure the diamonds are separated.
In a small saucepan, combine the remaining 1 3/4 cups of sugar with the water and bring to a boil. Simmer for 5 minutes, stirring to dissolve the sugar. Stir in the lemon juice. Pour the hot syrup evenly over the baklava and let cool for at least 1 hour before serving.