Preheat the oven to 375°F (190°C).
If using fresh spinach, wash and chop it coarsely. If using frozen spinach, squeeze out excess water after thawing.
In a frying pan over medium heat, add olive oil and sauté the chopped onion until translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the spinach to the pan and cook until wilted (if fresh) or heat through (if frozen). Remove from heat and let cool slightly.
In a large mixing bowl, combine the cooked spinach mixture with crumbled feta cheese, eggs, dill, parsley, salt, and pepper. Mix well until all ingredients are thoroughly combined.
On a clean surface, lay out a sheet of phyllo dough, brushing it lightly with melted butter. Layer with another sheet of phyllo dough and brush with more butter. Repeat this process until you have about 4-5 layers.
Spoon some of the spinach filling along one edge of the layered phyllo, then fold in the sides and roll it up tightly to form a log. Place the rolled spanakopita seam-side down on a baking sheet lined with parchment paper.
Repeat the process with the remaining phyllo dough and spinach filling.
Brush the tops of the rolled spanakopita with more melted butter.
Bake in the preheated oven for 50 minutes, or until golden brown and crispy.
Allow to cool slightly before slicing into pieces and serving.