Preheat the oven to 375F (190C). Lightly brush a 9x13-inch baking dish with olive oil.

In a large skillet over medium heat, saute the onions in a small amount of olive oil until translucent.

Add the chopped spinach to the skillet and cook until just wilted. Remove from heat and firmly squeeze out all excess liquid to prevent a soggy pie.

In a large mixing bowl, combine the cooked spinach and onions with the crumbled feta cheese, parsley, cherry tomatoes, beaten eggs, salt, and pepper. Mix until thoroughly incorporated.

Unroll the phyllo pastry and cover it with a damp kitchen towel to prevent drying. Place one sheet of phyllo in the prepared baking dish and brush lightly with olive oil. Repeat this process to layer half of the phyllo sheets.

Spread the spinach and feta filling evenly over the layered phyllo dough.

Layer the remaining phyllo sheets on top of the filling, brushing each individual sheet with olive oil, including the top layer.

Carefully score the top layers of the pie into squares or triangles using a sharp knife.

Bake for 40 to 45 minutes, or until the phyllo is golden brown and crispy. Allow the spanakopita to cool slightly before slicing and serving.