1. Heat a gas or charcoal grill to medium-high heat. Whisk together orange juice, honey, soy sauce and pepper in a bowl until well blended. Place in a heavy duty resealable plastic bag with pork tenderloin and leave to marinate in the fridge for at least 20 minutes and up to 12 hours.
2. While pork marinates, make the vinaigrette. Finely chop the shallot and add to a bowl with mustard, red wine vinegar, lemon juice and olive oil. Using a fork, blend mixture until fully emulsified. Finely chop the garlic and add to the dressing. Season with salt (in moderation) and pepper to taste. Mix again until combined and set aside.
3. Place pork in the center of the hot barbecue. Grill, turning occasionally, until pork is browned on all sides and reaches an internal temperature of 145°F on an instant read thermometer, about 15 minutes. After the first 5 minutes, add the green onions to the grill and cook until nicely charred. After 10 minutes, halve the plums, put them on the barbecue cut side down and sear about 2 minutes.
4. Remove the pork from the grill and leave to rest for 5 minutes loosely covered with foil. Dress the arugula with the shallot-vinaigrette and toss with the grilled plums. Slice the pork into medallions and place on the arugula with the grilled green onions. Serve immediately.
Per serving: 482 calories, 32g fat, 5g saturated fat, 70mg cholesterol, 561mg sodium, 27g carbohydrate, 2g fiber, 22g sugar, 25g protein