Galaktoboureko (Greek Custard-Filled Phyllo Dessert)

To make the syrup:
In a saucepan, bring the sugar and water to a boil over medium heat. Add the lemon slices, reduce heat to low, and simmer for 10 minutes. Remove from heat and remove lemon slices. Add the lemon juice and set aside to cool.
Make the filling:
Pour milk into a large saucepan, and bring to a boil over medium heat. When the milk comes to a boil, whisk in the semolina, sugar, eggs, lemon juice and lemon peel, stirring constantly. Cook, stirring constantly until the mixture thickens and comes to a full boil. Remove from heat, and set aside.
Preheat your oven to 350 F.
Butter a 9x13 inch baking dish, and layer 6 sheets of phyllo into the pan, brushing each one with melted butter as you lay it in. Pour the semolina custard into the pan over the phyllo, and cover with the remaining 6 sheets of phyllo, brushing each sheet with butter as you lay it down.
With a very sharp knife, cut into serving-sized pieces, cutting all the way through to the bottom. The phyllo and custard filling are delicate so cut with care and with a really sharp knife.
Bake for 40 minutes, until the top crust is crisp and golden brown and the custard filling has set.
Remove from the oven and pour the cooled syrup evenly over the top. Make sure you also pour around the edges. Allow Galaktoboureko to cool and absorb the syrup (around 4 hours).
Serve warm or cold. A scoop of vanilla ice cream goes really well with this too, especially when served slightly warmed!
Refrigerate leftovers.