Add sugar, honey, cinnamon stick, lemon juice, water, and citrus slices (optional) to a small saucepan. Simmer over medium heat until boiling. Turn the heat down to low, and continue to simmer for approximately 20 minutes. Set aside until needed.
Custard Filling
In a small saucepan, whisk together milk, eggs, vanilla, salt, sugar, and semolina. Cook over low-medium heat until custard mixture is thick and bubbly. Turn off stove, then add butter and stir until melted.
Assembly Instructions (Preheat oven to 375°F)
Melt ½ cup of butter, then use to brush to grease a 9"x13" ceramic dish.
Unroll phyllo dough and cover with a kitchen towel to prevent from drying out. Place one sheet into the pan, fold if necessary, and brush with melted butter. Repeat until you've used half of the sheets.
Use a rubber spatula to spread the cooled custard in an even layer over the phyllo sheets.
Cover with the remaining sheets, buttering each as you go. Use a sharp knife to score the pastry into squares, slicing only until you reach the custard. Slice each square into 2 triangles (see photos). Bake for 45 minutes, or until golden brown.
Remove Galaktoboureko from the oven. Strain syrup, then pour cool syrup over hot pastry. Cool for an hour before slicing all the way through.
Notes
Phyllo dough comes in various sizes. If the sheets are twice as large as your baking dish, simply fold them in half and add them to the pan. If the phyllo sheets are too long, alternate folding them first on one end of the pan, then the other.
Fillo dough can usually be found in the freezer section of most grocery stores. Make sure you defrost it for a few hours before using it.
Galaktoboureko is best when served fresh. The phyllo dough will go from crispy to soft the longer it sits. For this reason, I don't recommend making this dish ahead of time!
Leftovers of this Galaktoboureko recipe can be stored in an airtight container in the fridge for up to 3 days.