Place a 9-by-13-inch metal pan in the oven and preheat to 500 degrees . in a bowl, toss the first three ingredients; season. in a colander, toss the cucumbers with 3/4 tsp. salt; let sit in the sink to drain.
Cut a lengthwise slit 2 inches deep along the center of the tenderloin. Rub with EVOO and season. insert 5 or 6 plum wedges, skin side out. Drizzle the balsamic liquid over the plums. Close the pork over them, securing with toothpicks.
Transfer to the pan seam up; roast 8 minutes. Flip, add chopped plums to the pan and roast until pork registers 145 degrees on an instant-read thermometer, 10 to 14 minutes. Transfer to a cutting board.
Rinse the cucumbers and toss with the onion, lemon juice and 1 tbsp. EVOO. Season. Slice the pork and serve it with the saucy chopped plums and the cucumbers.