Season pork loin with salt and pepper, and place it in a large bowl or ziplock bag.
In a medium mixing bowl, whisk the olive oil, mustard, soy sauce, thyme, garlic, parsley, lemon juice, and orange juice.
Pour the marinade mixture over the roast, making sure to coat every inch.
Marinate it for at least an hour (overnight is ideal). You can leave it marinating in the fridge for up to 24 hours for optimal flavor.
When ready to cook, remove it from the marinade, shake off any excess, and discard the remaining marinade.
Preheat the oven to 400℉ (205℃).
In a large cast-iron skillet, heat the oil over medium heat until hot. Then, sear the roast for 5-6 minutes, rotating it 2-3 times to brown on all sides.
Transfer it to a roasting rack, and cook it in the oven, uncovered, at 400℉ (205℃) for 13-15 minutes. Turn it over halfway through for even cooking.
The right internal temperature will depend on your preference, but the FDA recommends around 145°F (63℃). Remove it from the oven, transfer the roast to a cutting board, and let it rest 5-10 minutes before slicing.