Can I add other greens to the spinach pastry stuffing? Yes, you can replace part of the spinach with oregano, collard green or kale.
Lemon juice or citric acid traditionally these spinach fatayer were made with lemon juice and consumed immediately. The problem with using lemon juice if that it makes the filling wet and this can cause the pastry triangles to open during baking or to turn slightly soggy if you store them for a couple of days. To get the tart taste without the extra fluid you can use citric acid. Citric acid is a week organic acid that is used to add a tart or sour taste to food and it is used to acidify milk in cheese making.
Tart or mild the use of sumac, citric acid or lemon juice is meant to add a tart taste to the spinach filling. You can make these spinach triangles without them and enjoy a mild fatayer. If you choose to use them, the amount is up to your taste. The amounts in the recipe are suggestions. Add them little by little and adjust the amounts according to your taste.
Shapes: You can shape these fatayer or pastry in any shape you like. Any form that encloses the filling is fine.
The size: you can make these triangles small if you plan on serving them to guests or at a party. You can also make them big if you like. It is up to your taste.