How to Make It
In a 2-gallon resealable plastic bag, combine the orange peel, smoked paprika, coriander and sumac. Add the pork, seal the bag and turn to coat it thoroughly with the spices. Refrigerate overnight.
Preheat the oven to 375°. On a large rimmed baking sheet, toss the sweet potatoes with 2 tablespoons of the olive oil and the rosemary; season with salt and pepper. Roast the potatoes, turning once, until tender and lightly browned, 40 minutes. Drizzle with the melted butter; keep warm.
Meanwhile, in a large ovenproof skillet, heat 2 tablespoons of the olive oil. Scrape most of the spice rub off the pork and season the meat with salt and pepper. Add the pork to the skillet and sear over moderately high heat, turning, until lightly browned all over, about 5 minutes. Transfer the skillet to the oven and roast for about 20 minutes, until an instant-read thermometer inserted in the thickest part registers 135°. Transfer the tenderloins to a cutting board and let stand for 10 minutes.
Spread the hazelnuts in a pie plate and toast until fragrant, about 12 minutes. Rub the hazelnuts together in a clean kitchen towel to remove the skins. Finely chop.
In a small bowl, mix the vinegar with the honey and the mustard. Whisk in the hazelnut and grapeseed oils, stir in the hazelnuts and season with salt and pepper.
Slice the pork on the bias. Arrange the slices on a platter along with the sweet potatoes and rosemary sprigs. Drizzle with the vinaigrette and serve.
Ground sumac is available at specialty stores and online at penzeys.com.