Place leeks and potatoes in a cast iron pan or baking dish. Drizzle with olive oil and season with salt and pepper. Toss.
Drizzle chicken all over with olive oil. Season chicken on both sides with paprika, salt and pepper (about 1 Tbsp of kosher salt and 1/2 tsp black pepper). Place on top of the leeks and potatoes, breast side up.
Place into preheated oven and cook until chicken is golden brown and the breast has reached an internal temperature of 160F (it will carry over the extra 5 degrees). This will take about 1 hr and 10 minutes.
As the chicken cooks, make your herb sauce by mixing all ingredients together in a small bowl. Set aside.
Once roasted, remove chicken from pan to rest. Let chicken rest for at least 15 minutes.
While chicken rests, place pan with potatoes and leeks under the broiler for a few minutes to brown.
Serve chicken and potatoes topped with herb sauce and enjoy! This is also lovely with a nice side salad!