Remove the thawed spinach from the package and put in a colander in the sink. Let it drain while you prepare the rest of the items.
Add the oil to a medium-sized frying pan over medium heat. Add the onion and cook for 5 minutes, stirring frequently.
Using a wooden spoon, press the spinach against the side of the colander to press out any remaining water.
Now add the drained spinach, garlic, parsley, dill, scallions, lemon zest, and feta cheese to the pan. Stir for 2 minutes to combine, then remove from heat and set aside.
Lay out the thawed phyllo dough stack and place it horizontally on a large cutting board. With a pizza cutter, cut the sheets into five 3-inch wide strips. The length should be around 12 inches. Once cut, place a damp tea towel over them to keep them from drying out.
On another board, pull out 3 phyllo strips and place them side by side. Brush them with butter, then top with another phyllo strip and butter them again.
Now add 1 tablespoon of the spinach mixture to the bottom of each strip. Fold over the corner to form a triangle. Fold again, keeping the triangle shape, and fold all the way up until you reach the top of the phyllo strip. Place the filled dough triangle on a baking sheet and keep this covered as you work with another damp tea towel.
Repeat this process until you have used up all of the filling.
Once the triangles are all laid out on the baking sheet, coat them with a little more butter on top.
Bake for 20-25 minutes, or until the tops are golden brown. Top with fresh dill and scallions if desired.