In a mixing bowl, combine well-drained, steamed spinach, finely chopped fresh herbs like mint, dill, and parsley, and sliced green onion. Crumble the feta cheese into the mixture.

Place a non-stick pan over medium heat and allow it to warm up for about 2 minutes.

Lay one tortilla flat in the pan. Sprinkle half the kasseri cheese evenly over the tortilla. Scoop the spinach and feta mixture onto the cheese. Top with the remaining kasseri cheese and place the second tortilla on top.

Let the quesadilla cook for about 2-4 minutes, or until the bottom tortilla turns golden brown. Gently lift the edge to check.

Using a large plate, carefully invert the quesadilla to flip it over. Slide it back into the pan and let it cook on the other side for another 2-4 minutes.

Once both sides are crispy and golden, transfer the quesadilla to a cutting board. Allow it to cool for a minute before slicing it into wedges.