Preheat oven to 425°F / 218°C. Line a sheet pan with foil.
Pat chicken thighs dry with paper towels. Set aside.
Whisk together all ingredients for dressing.
Place potatoes and rosemary sprigs in a large mixing bowl and toss with one third of the dressing. Spread potatoes out on prepared sheet pan.
Put chicken in mixing bowl and toss with one third of the dressing. Nestle chicken skin-side up in potatoes.
Season the skin of the chicken thighs with some additional salt and pepper.
Roast, stirring potatoes halfway through cooking, until potatoes are tender and chicken is cooked through and very crisp, 40 to 50 minutes. (Chicken is done when it registers 165°F in the thickest part.)
When chicken and potatoes are done roasting, drizzle with remaining one third of dressing. Garnish with a few fresh rosemary sprigs.