Place a large heavy baking sheet in oven. Preheat the oven to 400 degrees (keep the baking sheet in oven as it preheats). Combine potatoes, 1 1/2 tablespoons oil, garlic and thyme in a medium bowl; toss to coat. Arrange the potato mixture on a pre-heated baking sheet and sprinkle with 1/2 teaspoon salt and 1/2 teaspoon black pepper. Bake at 400 degrees for 30 minutes or until browned and tender, turning after 20 minutes. Combine remaining 1 1/2 tablespoons oil, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon black pepper, vinegar, shallots, Dijon mustard and tarragon in a small bowl, stirring well with a whisk. Drizzle dressing over potatoes.