Place oil in a large cast iron skillet over medium high heat. Pat fillet, shrimp and scallops dry. Season on all sides with salt. Place into hot skillet and cook for 2 minutes, flip and cook for another 2 minutes. Add marinara sauce and bring to a gentle boil. Simmer for 2 minutes to heat through. Garnish with pepper and serve.
Store extras in airtight container in the fridge for 4 days. Can be frozen for up to a month. To reheat, place in a skillet over medium heat for 5 minutes or until heated through.