Inside a pot under medium heat, add the Olive Oil (to taste), Garlic (2 clove), and Yellow Onion (1). Stir through for a while until the onions are softened.
After the onions are softened, add the Spinach Leaves (2 bunch). Stir for about 6 minutes until everything is mixed well. After six minutes, turn the heat off and transfer the spinach to a strainer so that excess liquid will drain off.
Transfer the spinach into a mixing bowl, and to that add the Feta Cheese (100 gram), Lemon Juice (1 tablespoon), and Salt and Pepper (to taste). Mix everything well. Set aside while you work on the pastry.
Brush over one of the Phyllo Dough (12) with melted Butter (100 gram). Add another phyllo on top and brush that with some butter as well. Finally cover everything with a third phyllo, and cut into three long strips of pastry.
Add a tablespoon of the spinach mixture over the edge of one of the strips. Keep folding over making a triangle each time until it is all wrapped up. Brush each triangle with some butter. Continue on with the rest of the strips.
Place the triangles on a baking tray, and bake inside a preheated oven under 180 degrees C (350 degrees F), for about 25 minutes.