Preheat oven to 425F. Move the rack to the middle. Cut your potatoes into halves and chop the sage.
To spatchcock your chicken: Place it breast-side down on the large baking sheet you plan on using for this recipe. Cut the backbone of the chicken out (cut on either side) using sharp kitchen shears. You can discard the backbone or save it for other purposes e.g. stock. Spread the chicken open Hannibal Lecter style. Flip it over (breast side up) and press firmly on the breastbone to flatten it out further. If you want step-by-step photos of this process, check out this article.
Add the potatoes to the baking sheet. Coat the potatoes with 2 tbsp of the oil, and rub the remaining 1 tbsp oil on the chicken. Season the chicken with a bit of garlic powder, smoked paprika, and salt & pepper. Season the potatoes with plenty of salt & pepper. Sprinkle the fresh herbs evenly over the chicken and potatoes (take the rosemary and thyme off their sticks if you wish... I took some off and threw in a few whole sprigs for good measure), then toss the potatoes (I just used my hands) to coat.
Roast for 45-55 minutes. My chicken was just under 3 pounds and I roasted it for 50 minutes. Giving it a few more minutes wouldn't have hurt, but be mindful that this process is much faster than the usual way of roasting chicken when adjusting cooking times. Chicken is done when a thermometer inserted into the thickest part of the breast is 165F. If you want the skin to crisp up even more, give it a blast under the broiler for a few minutes before you take it out of the oven (watch it carefully).
Let chicken rest for a few minutes, then carve it up and enjoy!