In a large skillet, saute onion and celery in oil, until tender. Add tomatoes, parsley, bay leaves, salt and pepper. Cover and simmer gently for 20 minutes. Add sole fillets and continue cooking for 10 minutes; or until fish flakes easily with a fork. Remove fish to a warm platter. If thicker sauce is desired, add a small amount of flour and cook until mixture is thickened and starts to boil.