Preheat the oven to 350°F.

In a large mixing bowl combine 3 oz softened cream cheese, ½ cup shredded mozzarella, ½ cup crumbled feta, and 3 oz roughly chopped baby spinach. Mix with your hands or a spatula until thoroughly combined and paste-like. Set the mixture aside.

Pat the 6 flounder fillets dry with paper towels and lay them flat on a large board. Measure the ½ tsp Old Bay seasoning and divide it in half.

Sprinkle half of the Old Bay (about ¼ tsp total) evenly over the surface of the fillets.

Divide the spinach-cheese mixture into 6 equal portions. Working with one fillet at a time, shape a portion of the stuffing into an oval/egg shape, place it near the tail end of the fillet, then roll the fillet up from the tail toward the head jelly-roll style. Place each rolled fillet seam side down in a single layer in a baking dish.

Sprinkle the remaining half of the Old Bay (about ¼ tsp) evenly over the tops of the rolled fillets. Using a pastry brush or spoon, brush the 1 tbsp olive oil evenly over the tops.

Bake in the preheated oven for about 20 minutes, until the fish is opaque and flakes easily with a fork.