Heat oven to 350 degrees. Line a sheet pan with parchment paper and lightly brush the paper with about 2 teaspoons of olive oil. Pat pork tenderloin dry with paper towels and place on the sheet pan, arranging apple, onion, fennel slices, and potato around the pork.
In a small bowl which together remaining olive oil, white balsamic vinegar, garlic, Dijon, herbs, salt and pepper. Brush the pork tenderloin and vegetables with the marinade, leaving roughly 4 tablespoons for later.
Roast for 35 - 40 minutes or until thermometer inserted in the center of the pork registers 145 degrees. Remove pork to rest on a cutting board. Increase the oven temperature to 400 degrees. Scatter chard over the vegetables and brush all the remaining marinade. Using a wooden spoon or plastic spatula (so as not to tear the parchment), give all the vegetables a good toss.
Roast an additional 5-10 minutes or until tender, taking care not to over cook the chard. Serve pork sliced, surrounded by vegetables.
