Mix flour, yeast, sugar, and salt together. Add warm water and olive oil slowly.
Knead for 8-10 minutes until smooth. The dough should feel soft and elastic.
Cover with a towel and let it rise. Wait about 1 hour until it doubles.
Heat olive oil in a pan over medium heat. Add diced onion and cook until soft.
Toss in chopped spinach and cook briefly. Add lemon juice, sumac, salt, and pepper.
Mix everything well and remove from heat. Let the filling cool down completely.
Divide dough into 12 equal balls. Roll each ball into a small circle.
The circles should be about 4 inches wide. Place 1 tablespoon of filling in the center.
Fold three sides up to form a triangle. Pinch the edges tightly to seal them.
Preheat your oven to 375°F (190°C). Place fatayer on a lined baking sheet.
Brush each one with beaten egg gently. Sprinkle sesame seeds on top if desired.
Bake for 20-25 minutes until golden brown. Remove and let them cool for 5 minutes.