In a large skillet, heat the olive oil over medium heat. Add the sliced onion and diced red bell pepper, cooking until they become soft and translucent (about 5-7 minutes).
Add the chopped spinach to the skillet and cook until wilted (about 2-3 minutes). Stir in the garlic powder, ground cumin, salt, and pepper, mixing well. Remove from heat and let it cool slightly.
In a mixing bowl, combine the cooked vegetable mixture with the crumbled feta and shredded mozzarella cheese. Mix until well combined.
Wipe the skillet clean and return it to medium heat. Place one flour tortilla in the skillet and spread a generous portion of the spinach and cheese mixture over half of the tortilla.
Fold the tortilla in half to cover the filling and cook for about 3-4 minutes per side, or until golden brown and the cheese has melted. Repeat with the remaining tortillas and filling.
Once all quesadillas are cooked, cut them into wedges.
Garnish with chopped fresh parsley before serving.