Preheat your oven to 425°F / 215°C and place a rack in the lower third. Lightly grease a large baking sheet with cooking spray.
In a skillet over medium heat, warm the olive oil.
Add the finely chopped onion and sauté for about 3 minutes until soft and translucent, but not browned.
Transfer the cooked onions to a medium bowl.
Add the squeezed spinach, lemon juice, toasted pine nuts, fresh parsley, sumac, sea salt, and black pepper.
Stir until everything is thoroughly combined.
Divide the pizza dough into 10 equal portions. On a lightly floured surface, roll each portion into a 5-inch / 13-cm circle.
Spoon about 3 tablespoons (45 ml / 1.5 fl oz) of the spinach filling into the center of each dough circle. To form a triangle, bring three sides of the dough up to the center and pinch them together firmly to seal the top.
Continue pinching along the three seams to ensure the pie is completely sealed.
Place the assembled fatayer on the prepared baking sheet.
Bake for 15 minutes, or until the dough is golden brown and cooked through.
Serve warm.
Don't overfill the dough.
Using about 3 tablespoons (45 ml / 1.5 fl oz) of filling per triangle will make them easier to seal and prevent them from bursting open while baking.