In a large bowl, combine the flour and salt.
Make a well in the center and add the warm water, vegetable oil, and egg.
Mix until a dough forms. Knead on a floured surface for about 5 minutes until smooth and elastic.
Cover the dough with a damp cloth and let it rest for 30 minutes.
Heat the olive oil in a pan over medium heat. Add the chopped onions and sauté until translucent.
In a bowl, mix the feta cheese, ricotta cheese, cooked spinach, sautéed onions, egg, black pepper, and nutmeg.
Divide the dough into 2 equal parts.
On a floured surface, roll out one part of the dough into a thin rectangle (about 12x16 inches).
Brush the rolled dough with melted butter or oil.
Spread half of the filling evenly over the dough.
Roll the dough up tightly from one end to form a log.
Place the rolled dough onto a greased baking sheet and shape it into a spiral or coil.
Repeat with the second part of the dough and remaining filling.
Preheat your oven to 375°F (190°C).
Brush the top of the burek with additional melted butter or oil.
Bake for 30-35 minutes, or until golden brown and crispy.
Allow the burek to cool slightly before slicing.
Enjoy your meal!