Prepare the Filling
In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes. Add the garlic and cook for another minute until fragrant.
Stir in the chopped spinach and cook until wilted, about 2-3 minutes. Remove the skillet from the heat and let the mixture cool slightly.
In a mixing bowl, combine the cooked spinach mixture with the crumbled feta cheese, cream cheese (if using), oregano, lemon juice, salt, and pepper. Stir until well combined.
Stuff the Flounder
Preheat your oven to 375°F (190°C).
Place the flounder fillets on a baking sheet lined with parchment paper or lightly greased. Season the fillets with salt and pepper on both sides.
Spoon the spinach and feta mixture onto the center of each flounder fillet. Carefully fold the edges of the fillet over the stuffing, creating a pocket. Use toothpicks to secure the edges if necessary.
Bake the Flounder
Drizzle the remaining tablespoon of olive oil over the top of the stuffed flounder fillets.
Bake the stuffed flounder for 15-20 minutes, or until the fish is cooked through and flakes easily with a fork. The filling should be hot and the fish golden brown on the outside.
Serve
Remove the toothpicks and transfer the stuffed flounder to serving plates. Garnish with fresh parsley or dill, if desired, and serve with lemon wedges for an extra burst of flavor.