Preheat a grill to medium high. Butterfly the pork tenderloin: Slice lengthwise down the middle of the pork, cutting about two-thirds of the way through, and open like a book. Pound the pork with a meat mallet or heavy skillet until about 3/4 inch thick. Season with salt and pepper. Mix the fennel, garlic, lemon zest, red pepper flakes and 2 tablespoons olive oil in a small bowl; rub the mixture all over the pork and set aside, 10 minutes.
Meanwhile, bring a large pot of salted water to a boil and fill a large bowl with ice water. Add the cabbage to the boiling water and cook until bright green and just tender, about 2 minutes. Drain and transfer to the ice water to stop the cooking. When cool, drain the cabbage again and blot very dry with kitchen towels. Combine the cabbage, lemon juice, parsley, walnuts, Parmesan and remaining 1/4 cup olive oil in a large bowl. Toss well and season with salt and pepper.
Grill the pork until well marked and just cooked through, 4 to 5 minutes per side. Transfer to a cutting board. Divide the cabbage salad among plates. Slice the pork and arrange on top; season with pepper.