Make the turkey tenderloin: Preheat the oven to 375°F (190°C).
2. Step
In a small bowl, mix together the onion powder, garlic powder, sage, thyme, brown sugar, nutmeg, and salt.
3. Step
Using your hands, rub each turkey tenderloin with 1 teaspoon vegetable oil, then generously coat all over with the spice rub.
4. Step
Heat the remaining tablespoon of oil in a 12-inch cast iron or oven-safe skillet over medium-high heat until it begins to shimmer. Add the turkey tenderloins to the skillet and sear for 3 minutes on each side, until golden brown. Remove the pan from heat and pour in the chicken stock.
5. Step
Cover the pan tightly with foil and transfer to the oven. Bake for 20–25 minutes, or until the turkey is cooked through and a thermometer inserted into the thickest part of the tenderloin registers at least 165°F (74°C). Transfer the turkey from the pan to a cutting board, tent with foil, and let rest for at least 10 minutes, or until ready to serve. Leave any juices behind in the pan.
6. Step
While the turkey roasts, make the creamy cranberry sauce: In a small saucepan, combine the cranberries, orange peels, orange juice, brown sugar, rosemary, and water. Bring to a boil over medium heat, then reduce the heat to medium-low and simmer for 8–10 minutes, until the cranberries have softened and burst and the liquid has thickened. Remove the pot from the heat and let cool for 10 minutes. Remove the orange peels and rosemary sprig and discard.
7. Step
Transfer the cranberry mixture to a small food processor, along with the mayonnaise and salt. Pulse until smooth. Transfer to a small bowl and refrigerate until ready to use.
8. Step
Make the jus: Return the skillet used to roast the turkey to the stove and melt the butter over medium heat. Add the flour and whisk to incorporate. Cook for 3–4 minutes, until bubbling and thickened. Add the chicken stock and bring to a boil, then reduce the heat to medium-low and simmer for 8–10 minutes, until thickened. Season with the salt
9. Step
Set a fine-mesh sieve over a measuring cup or serving bowl and strain the jus to remove any solid bits.
10. Step
When ready to serve, thinly slice the turkey against the grain.
11. Step
Spread about ½ tablespoon of softened butter on each side of the brioche buns. Working in batches if needed, place the buns, buttered-side down, in a clean large skillet over medium heat and toast until the buns are golden brown, about 4 minutes.
12. Step
Assemble the sandwiches: Spread 1–2 teaspoons of creamy cranberry sauce over each side of the buns. Place ½ cup arugula, 6 ounces turkey, and ⅓ cup mac ‘n’ cheese on each bottom bun and top with the top buns. Serve with sweet potato fries, more cranberry sauce, and the hot jus for dipping.
