Saverd at: epicurious.com
Place a rack in top third of oven; preheat to 375°F. Whisk Dijon mustard, 2 Tbsp. honey, and 1 Tbsp. oil in a medium bowl to combine. Season chicken on all sides with 1 tsp. salt and 1/2 tsp. pepper. Transfer to bowl with honey mustard and toss to coat.
Toss potatoes, 2 Tbsp. oil, 1 tsp. salt, and remaining 1/2 tsp. pepper on a rimmed baking sheet. Arrange chicken legs (skin side up) in the center of pan; discard excess marinade. Arrange potatoes around outside of pan in an even layer, then turn cut side down (this will help them brown better). Arrange rosemary over potatoes. Roast until chicken and potatoes are golden brown and an instant-read thermometer inserted into thickest part of chicken registers 165°F, 30–40 minutes.
Whisk garlic, lemon juice, whole grain mustard, and remaining 1/3 cup oil, 2 tsp. honey, and 1/2 tsp. salt in a small bowl. Divide chicken and potatoes among plates, then arrange frisée and parsley alongside potatoes. Drizzle dressing over chicken, potatoes, and frisée.
Do Ahead: Chicken can be marinated 8 hours ahead. Cover and chill.