Preheat oven to 425°F. While you wait, slice 3 plums and 1 fennel bulb, aiming for ½-inch thick slices. Transfer cut fruit and vegetables to a sheet pan and cover with a generous pinch of salt and enough olive oil to lightly coat. Push everything to the perimeter of the tray, leaving an opening in the middle.
To the opening add 500g pork tenderloin and cover with 2 tablespoon balsamic vinegar, ½ tablespoon grain mustard and ½ tablespoon dijon mustard. Toss to coat, then spread any extra marinade onto the plums and fennel. Transfer to the oven and roast for 20 minutes. Meanwhile, prep 1 cup farro, lentils, or other grain/legume for serving. A nice piece of crusty bread will also work!
When the plums are jammy and the fennel is caramelized, position the rack in the upper third section of the oven and turn to broil. Let broil for 5 minutes, or until a crust starts to form on the pork (it should have an internal temperature of 145°F-155°F. Remove and let rest for 5 minutes.
Slice the pork into rounds and serve with plums, fennel and farro topped with any leftover fennel fronds!
