This assumes you know how to put together the borek with the filo dough!
You will need a large clean workspace for this.
Set oven to 350º
Take large cookie sheet with edges and set to the side.
Remove filo from package. Lay flat and cover with slightly damp think kitchen towel.
Saute 1 chopped onion in oil, add spinach (drained really well in colander before adding!),
Remove from heat.
Stir in cheeses, eggs, sugar, and 4 T milk and remove from heat.
In a separate small bowl, melt 1 cube butter (in microwave, covered ok)
In another separate bowl, whisk lightly, 1 egg with 1/2 c milk.
Now you can begin the layering of the borek!
I prefer to not smooth out my filo layers and sometimes end up with 2 in one layer. Add/drizzle a small amount of the butter and then the milk/egg mixture by tablespoons onto each layer. Once you've place 1/2 the filo sheets in this manner, add the filling. Continue the layering, when finished, place in oven for 30-40 min until golden brown on top.