In a medium bowl combine the vinegar, apple juice concentrate, and honey. Slowly add the oil, whisking constantly until the dressing is emulsified. Season with the salt and pepper. Transfer ⅓ cup of the dressing to a small bowl and reserve for the salad.
Place the tenderloins a large, resealable plastic bag and pour in the remaining dressing. Press the air out of the bag and seal tightly. Turn the bag to distribute the dressing, place in a large bowl, and refrigerate for 2 to 4 hours, turning occasionally. Meanwhile, prepare the walnuts.
In a medium skillet combine the brown sugar and vinegar. Bring to simmer over medium heat, stirring until the sugar dissolves. Add the walnuts and cook until the nuts are completely coated with the sugar mixture and there is no longer any sugar mixture left in the skillet, 2 to 4 minutes, stirring constantly. Transfer the nuts to a plate and let cool completely. (The nuts can be prepared in advance and kept in an airtight container at room temperature for up to 2 days.)
Prepare the grill for direct cooking over medium heat (350° to 450°F).
Remove the tenderloins from the bag and reserve the dressing. Season the tenderloins evenly with the salt, pepper, and thyme. Pour the dressing from the bag into a small saucepan and bring to a boil over high heat. Boil for 1 minute. Remove from the heat and set aside for brushing over the grilled tenderloins.
Brush the cooking grates clean. Grill the tenderloins over direct medium heat,with the lid closed as much as possible, until the outside is evenly seared and the internal temperature reaches 150°F, 20 to 25 minutes, turning occasionally. Remove from the grill and let rest for 3 to 5 minutes. Brush the tenderloins evenly with the boiled dressing.
In large bowl toss the frisée, apple, cranberries, and candied walnuts. Add the reserved dressing and toss to coat.
Cut the tenderloins crosswise into ½-inch slices. Divide the slices among four plates and arrange the salad alongside. Serve immediately.