Preheat the oven to 350°F. Lightly butter or grease a baking dish large enough to hold all six fillets.

Place the drained crabmeat in a medium bowl. Add the breadcrumbs and gently toss to coat the seafood.

In a separate small bowl, whisk together the mayonnaise, egg, Old Bay seasoning, parsley, juice from half of the lemon, salt and pepper.

Pour the mayonnaise mixture over the seafood. Gently fold everything together until evenly combined. Avoid overmixing so the crabmeat does not break apart too much.

Lay the flounder fillets flat on a clean work surface. Divide the seafood stuffing evenly among the fillets, placing it near the center of each piece of fish.

Carefully roll each fillet around the filling. Arrange the rolled fillets seam-side down in the prepared baking dish. If a fillet is too small or thin to roll, fold it over the filling instead.

Combine the melted butter with the juice from the remaining lemon half. Brush the lemon butter over the stuffed fillets, then sprinkle lightly with paprika.

Bake for 20–25 minutes, or until the flounder is opaque, flakes easily with a fork and reaches an internal temperature of 145°F. Thinner fillets may finish sooner, so begin checking them at 15 minutes.

Serve warm with lemon wedges, if desired.