Wash and dry spinach, chop in thin strips. Mix with crumbled feta cheese, onion and pepper flakes. Set aside.
Dough
In a medium size bowl mix together all ingredients. Knead for 5-8 minutes or until you obtain a smooth and soft dough.
Divide the dough into two equal parts. Roll each part into cylindrical shape and subdivide to 5 equal sections. Roll all 10 pieces into a dough ball each, lightly dust and cover with plastic film.
Melt the butter in a small sauce pan.
Dust the counter top with flour. Roll the ball into 15 cm (6″ ) disc shape and put it on a small plate. brush the top of the disc with melted butter.
Repeat the same process until a stack of 5 disk layers is formed, brush with melted butter the top of the last disk. Cover with plastic film and refrigerate for 20 minutes.
Repeat the same process with other 5 dough balls and make another stack of 5 butter brushed disks. Cover with plastic film and refrigerate 20 minutes.
Preheat the oven to 180ºC (355ºF) rack in the lower third.
Assembly and Baking
Lightly beat egg and mix with little melted butter.
Lightly butter 40cm (16″)baking pan.
Take one plate from the refrigerator and turn the dough stack onto lightly dusted counter. Roll out the dough to the size of baking pan. Transfer the dough to the pan.
Spread the filling evenly across the dough.
Take another plate from the refrigerator and turn the dough stack onto lightly dusted counter. Roll out the dough to the size of baking pan. Transfer the dough and place on top of the filling, seal the edge to the bottom dough.
With sharp knife and using a cup as a guide cut approximately 10 cm (4″) circle in the middle, divide the rest of the surface outside of the circle to 8 equal parts.
Brush the burek with mixture of egg and butter and bake for 50-55 minutes or until golden brown.