In a large bowl, combine the flour and salt. Create a well in the center and add the warm water, vegetable oil, and egg.
Mix until a dough forms. Knead the dough on a floured surface for about 5 minutes until it is smooth and elastic.
Cover the dough with a damp cloth and let it rest for 30 minutes.
In a medium bowl, mix together the feta cheese, ricotta cheese, mozzarella cheese, chopped parsley, and finely chopped onion. Season with salt and pepper to taste.
Preheat your oven to 375°F (190°C).
Divide the dough into 2 equal portions. On a floured surface, roll out one portion of the dough as thinly as possible, aiming for a large rectangle.
Brush a baking dish with a little vegetable oil. Place the rolled-out dough into the dish.
Spread the cheese filling evenly over the dough.
Roll out the second portion of dough and place it over the filling. Press the edges to seal.
Brush the top with beaten egg for a golden finish.
Bake in the preheated oven for 30-35 minutes, or until the burek is golden brown and crispy.
Allow the burek to cool slightly before slicing. Serve warm or at room temperature.
Enjoy your meal!