Thaw phyllo dough overnight in the refrigerator. Let sit at room temperature for 1 hour before using. Keep covered with a lightly damp towel while working.
Trim phyllo sheets to fit a 13x9 inch baking pan. Keep unused sheets covered.
Brush the baking pan with melted butter. Line with parchment if desired and brush again.
In a saucepan, combine sugar, water, honey, and lemon juice. Bring to a boil, then reduce heat and simmer 4 minutes. Cool completely.
Pulse walnuts until finely chopped. Mix with cinnamon.
Preheat oven to 325°F (163°C).
Layer 10 sheets of phyllo in the pan, brushing each with melted butter. Spread one-fifth of the nut mixture evenly over the top.
Add 5 buttered sheets, then another portion of nuts. Repeat layering four times. Finish with 10 buttered sheets on top.
Cut into strips and diagonally into diamond shapes before baking.
Bake for 1 hour and 15 minutes until deep golden brown and crisp.
Immediately spoon cooled syrup evenly over hot baklava. Listen for a gentle sizzle.
Let rest uncovered at room temperature for 4 to 6 hours or overnight before serving. Garnish if desired.