Preheat oven to 375 degrees F (190 degrees C).

In a large skillet, heat olive oil over medium heat. Add diced onion and minced garlic, and cook until onion is translucent.

Add sliced mushrooms to the skillet and cook until softened. Stir in fresh spinach and cook until wilted. Remove from heat.

In a small bowl, combine grated Parmesan cheese and bread crumbs. Season with salt and pepper.

Lay flounder fillets flat on a clean surface. Spoon the mushroom and spinach mixture onto each fillet. Sprinkle the Parmesan and bread crumb mixture on top.

Roll up the fillets and secure with toothpicks. Place the stuffed flounder in a baking dish.

Bake in the preheated oven for 20-25 minutes, or until fish flakes easily with a fork.

Serve hot and enjoy!