Preheat your oven to 425°F (220°C). Line a large, rimmed baking sheet with parchment paper.
Place the chicken breast-side down on a cutting board. Using sturdy kitchen shears, cut along both sides of the backbone to remove it. Flip the chicken over and press firmly on the breastbone with the heel of your hand until you feel a crack and the chicken lies flat.
In a large bowl, whisk together the lemon juice, olive oil, Dijon mustard, grated garlic, 1 teaspoon of the salt, black pepper, garlic powder, paprika, oregano, and turmeric.
Pat the chicken thoroughly dry with paper towels. Place the chicken in the bowl with the marinade and use your hands to rub it all over, including under the skin. Place the chicken skin-side up on the prepared baking sheet.
Cut the potatoes into 1/2-inch (1.3 cm) thick rounds. Add the potato slices to the bowl with the remaining marinade, toss to coat, and season with the remaining 1/2 teaspoon of salt. Arrange the potatoes in a single layer around the chicken on the baking sheet.
Roast for 45 to 55 minutes. The chicken is cooked when the skin is golden and an instant-read thermometer inserted into the thickest part of the breast reads 165°F (74°C).
Transfer the chicken to a clean cutting board and let it rest for 10 minutes.
While the chicken rests, use a spatula to toss the potatoes on the baking sheet. Return the pan to the oven and cook for another 10 to 15 minutes, or until they are browned and tender.
Carve the rested chicken and serve immediately with the roasted potatoes.