Place the cream cheese, mozzarella, feta, and chopped baby spinach into a large mixing bowl. Massage the spinach and the cheeses with your hands until they are well combined (it should look like a thick paste). Set aside.
Place the flounder fillets onto a large board. Sprinkle the desired amount of Old Bay Seasoning onto each fillet.
Divide the stuffing into 6 equal portions. Working one at a time, shape the portion of the stuffing into an egg shape and place it close to the tail of the fish. Then starting from the tail, roll each fillet up jelly-roll style and place it seam side down on the baking dish.
Sprinkle some more Old Bay seasoning on top, and using a pastry brush, spread some olive oil over the fillets.
Bake at 350ºF for about 20 minutes, until the fish is cooked through.
