Preheat the oven to 375 degrees. Heat the oil in a large ovenproof skillet over high heat. Season the pork with salt and pepper and sear on all sides until golden brown. Combine the mustard and vinegar and brush over the pork. Transfer to the oven and roast the pork until a thermometer inserted in the center reads 145 degrees, about 35 minutes.
In a medium saucepan, heat olive oil and sauté garlic and red onions. Add 1 ¾ cups water and pearled couscous. Bring to a boil. Reduce heat to a medium and simmer for 8 minutes uncovered. Stir occasionally. Add rest of ingredients and stir. Bring back to a medium boil and simmer for 4-6 minutes or until couscous is tender. Let stand covered 4-5 minutes before serving. Stir well and serve immediately.