*Tip: If a phyllo sheet tears as you work, simply patch it together with melted butter, saving full sheets for top & bottom stacks.
1 1/4 cups (2 1/2 sticks) butter, melted, plus more for pan
3 1/2 cups sugar
2 Tbls fresh lemon juice
3 cups walnuts
1 1/2 tsp ground cinnamon
16 oz package frozen phyllo dough (17 x 12″), thawed in refrigerator
1. Heat oven to 375. Brush a 13 x 9″ baking dish with butter set aside. In a medium saucepan, combine 3 cups sugar, lemon juice, and 1 1/2 cups water; bring to a boil. Reduce heat; simmer until sugar dissolves and mixture is syrup, about 10 minutes. Set syrup aside.
2. In a food processor, pulse walnuts with cinnamon and remaining 1/2 cup sugar until finely ground. Set walnut filling aside.
3. Place stack of thawed phyllo sheets on a work surface . Using a sharp knife, trim stack into a 13 x 9 rectangle; discard trimmings. Place 1 sheet of trimmed phyllo in prepared baking dish (keep remaining sheets covered with a damp cloth or paper towel). Brush gently with butter; repeat with two more sheets of phyllo, laying each on top of the other. (You now have a 3-sheet stack of buttered phyllo.)
4. Sprinkle phyllo stack in dish with 1/3 cup walnut filling. Repeat with seven more buttered phyllo stacks, sprinkling each with walnut filling. Top with one more stack; brush generously with butter.
5. Using a sharp knife, cut unbaked baklava into 24 squares. Bake until puffed and golden, 30-35 minutes. Transfer pan to a wire rack; pour syrup over baklava. Let stand at room temperature until syrup is absorbed, at least 3 hours. To store, keep at room temperature, up tp 3 days.