Preheat the oven to 350 degrees F. Butter the bottom of a 9- by 13-inch pan.
Mix together the ground nuts, graham crackers and cinnamon in a bowl. Spread a phyllo sheet in the buttered pan and brush on some more butter with a pastry brush. Put down another sheet and repeat this process with 10 sheets, buttering each sheet. Sprinkle 1/4 of the nut mixture on top of the 10th sheet.
Layer on 4 more phyllo sheets, buttering each sheet, and top the last sheet with another 1/4 of the nut mixture. Repeat with 2 more layers of 4 phyllo sheets and 1/4 nut mixture, buttering each sheet. Top with 10 more sheets of buttered phyllo, adding lots of butter to the top of last sheet.
Before baking, cut into diamond shapes with a sawing action using a sharp knife as phyllo is fragile and will tear easily. Bake on the lower rack of the oven until golden brown, 1 hour.
For the syrup: While the baklava is baking, combine the sugar, honey and 1 1/2 cups water in a saucepan. Bring to a boil and let cook for 10 to 15 minutes, and then add the lemon juice and boil for a few minutes longer. Remove the syrup from the heat and let the mixture cool slightly.
Let the baklava cool slightly. Pour the honey mixture on top -- both syrup and baklava should be warm while doing this. Let the baklava and syrup cool thoroughly, uncovered. Let the syrup soak into the baklava for at least 6 hours or overnight.