Pat the roast dry. 2. Rub with tallow, salt, and pepper. 3. Refrigerate for 24 hours. 4. Preheat oven to 225F. 5. Roast until 120F internal. 6. Mix butter, marrow, and parsley. 7. Increase heat to 500F. 8. Sear for 10 minutes. 9. Rest for 20 minutes. 10. Whisk cream ingredients. 11. Serve.