1. Preheat oven to 400°F. Finely chop rosemary and garlic together and gather into a pile on a cutting board. Sprinkle rosemary-garlic mixture with salt and 1/4 tsp. pepper and grind with flat side of a chef's knife to form a paste. Transfer herb paste to a medium bowl along with 1 Tbsp. oil; add potatoes and toss until well coated.
2. On a large foil-lined rimmed baking sheet, arrange potatoes in a single layer. Roast, tossing once, until golden brown and tender, about 30 minutes.
3. On a serving plate, combine frisée, parsley, basil and mint. In a small bowl, whisk vinegar, mustard and remaining oil. Just before serving, drizzle salad with dressing and toss. Scatter potatoes over salad and serve.