Prepare the vinaigrette: Combine the mustard, tarragon, vinegar, salt and pepper in a small bowl. Slowly whisk in the oil. Correct the seasonings, including the mustard and vinegar, and set aside.

In a bowl, let the cheese marinate in the olive oil for up to an hour with 1 teaspoon of the thyme leaves, the garlic and pepper.

Wash the radicchio and arugola and dry in a salad spinner or pat dry with clean paper towels.

In a bowl, toss the salad greens with enough of the vinaigrette to coat them lightly. Divide the greens among 4 large salad plates.

Heat a non-stick saute pan until very hot and add 2 tablespoons of the goat cheese marinade. Add the cheese and saute it over medium heat for 30 seconds on each side.

Top each salad with a slice of hot goat cheese. Garnish the cheese with the remaining thyme leaves. Serve immediately.