• Preheat the grill to medium high.

For the vinaigrette:

• In a medium size bowl, add the minced shallot, vinegar, salt, tarragon and mustard. Add olive oil in thin steady stream whisking as you pour to emulsify. Set it aside.

For the salad:

• Place the potatoes in a large piece of aluminum foil, drizzle with 1 tablespoon of the olive oil, 1/4 teaspoon of the salt, and 1/4 teaspoon of the pepper. Then fold and seal the potatoes, as if creating a large aluminum foil package or envelope. Put it directly on the grill and cook for 30 minutes, flipping it once.
• In a large bowl, season the radicchio with the remaining 2 tablespoons of olive oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Grill the radicchio for about 3 minutes per side. Remove from the grill and cut it into 1/2-inch slices.
• Remove the potatoes from the grill, opening the package carefully as the steam will be piping hot. Transfer to a salad bowl, combine with the cut radicchio and toss with the prepared vinaigrette.