Whisk apple juice concentrate, 2 tablespoons mustard, 1 tablespoon oil, rosemary (or thyme), garlic and peppercorns in a small bowl. Reserve 3 tablespoons marinade for basting. Place tenderloins in a shallow glass dish and pour the remaining marinade over them, turning to coat. Cover and marinate in the refrigerator for at least 20 minutes or for up to 2 hours, turning several times.
Heat a grill or broiler.
Combine shallot, port (or tea), vinegar, salt, pepper and the remaining 1 1/2 teaspoons mustard and 1 tablespoon oil in a small bowl or a jar with a tight-fitting lid; whisk or shake until blended. Set aside.
Grill or broil the tenderloins, turning several times and basting the browned sides with the reserved marinade, until just cooked through, 15 to 20 minutes. (An instant-read thermometer inserted in the center should register 155°F. The temperature will increase to 160° during resting.)
Transfer the tenderloins to a clean cutting board, tent with foil and let them rest for about 5 minutes before carving them into 1/2-inch-thick slices. Arrange the pork slices on plates and drizzle with the shallot dressing. Serve immediately.
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