Preheat oven to 425 degrees. Place chicken, breast side up, on a baking sheet. Rub with 2 tablespoons oil; season generously with salt and pepper. Place parsley and 1 lemon half in cavity. Tie legs together with kitchen twine.
Toss potatoes and 2 tablespoons oil. Drizzle with juice from remaining lemon half. Season with salt and pepper. Scatter potatoes along chicken.
Roast for 15 minutes. Reduce temperature to 375 degrees; cook for 25 minutes. Rotate pan, toss potatoes, and cook until potatoes and chicken are golden brown and a thermometer inserted into the thickest part of the chicken's thigh reaches 165 degrees, about 25 minutes more. Let chicken and potatoes stand for 10 minutes.
Meanwhile, make the sauce: Combine ingredients. After 10 minutes, brush sauce on chicken and drizzle on potatoes.
Eh. The top of the chicken got nice and crispy but the bottom (and bottom of the sides), the skin limp and soggy from being right on the pan. I found the sauce very oily. Flavor was okay, but nothing exceptional. I'll be going back to my Cooks Illustrated roast chicken method.
I really loved this sauce. Not so much a fan of roasting it all together. I had about an inch of juice in the pan so the potatoes were not crisp. Definitely make the sauce though. Here are my results: http://wp.me/plYaL-1E8