Ensure the phyllo dough is thawed overnight in the refrigerator. An hour before starting, place it on the counter to reach room temperature. Trim the phyllo sheets to fit a 13x9 baking dish.
In a saucepan, combine sugar, honey, lemon juice, and water. Bring to a boil on medium-high heat, stirring until sugar dissolves. Reduce heat to medium-low and let it simmer for 4 minutes without stirring. Set aside to cool.
In a food processor, pulse the walnuts until they are coarsely ground. Transfer to a bowl and mix with cinnamon.
Preheat the oven to 325°F. Butter the bottom and sides of your baking dish. Begin layering 10 sheets of phyllo in the dish, brushing each with melted butter before adding the next. Spread about 3/4 cup of the walnut mixture over the sheets.
Add 5 more buttered phyllo sheets over the nuts. Repeat this process 4 times. Finish with a final layer of 10 buttered phyllo sheets on top. Brush the topmost layer with butter.
Slice the baklava into 1 1/2" wide strips. Then, make diagonal cuts to create diamond shapes.
Place the dish in the oven and bake for 1 hour and 15 minutes, or until the baklava turns a golden brown.
Once out of the oven, immediately pour the cooled syrup over the baklava, ensuring it seeps into the cuts.
Allow the baklava to cool completely at room temperature. For optimal flavor, let it sit for 4-6 hours or overnight. Optionally, drizzle with melted chocolate and sprinkle with chopped walnuts.
