Layer the onions, bell pepper and okra in a large, lined slow cooker. Pour in the marinara. Cover and cook on high until the vegetables are tender, about 3 1/2 hours.
Stir in the shrimp, scallops and crushed red pepper; drizzle with the EVOO. Cover and cook until the seafood is firm, about 15 minutes. Stir in the basil; season with salt and pepper.
Divide the slices of grilled polenta among plates. Top with the seafood mixture.