Rinse fish thoroughly in cold water; pat dry. Sprinkle cavity with salt, pepper, and lemon juice. Set aside.
Sauté mushrooms, celery, and onion in olive oil in a large skillet until tender. Stir in breadcrumbs, pimiento, and seafood seasoning. Stuff cavity of fish with mushroom mixture. Place fish in a lightly greased shallow roasting pan.
Bake at 350° for 35 minutes or until fish flakes easily when tested with a fork. Carefully remove and discard skin from top of fish. Transfer fish to a serving platter. Garnish with lemon slices.