Heat your oven to 375°F (190°C). Pat the chicken dry using paper towels so the skin roasts nicely. Brush the chicken all over with olive oil, then sprinkle the kosher salt evenly. Squeeze the juice from half the lemon and half the orange over the chicken. Place the remaining citrus halves and a few onion wedges inside the cavity for extra flavor.

Arrange the cubed potatoes, sweet potatoes, and the rest of the onion wedges around the chicken in a large roasting pan. Sprinkle the sugar over the vegetables, then carefully pour the sweet red wine into the pan, keeping it away from the top of the chicken. Gently stir the vegetables so they are lightly coated in the wine and cooking juices.

Roast the chicken uncovered for 1 hour 10 to 15 minutes. Baste the chicken once or twice during cooking using the juices from the pan to keep it moist. Stir the vegetables halfway through so they cook and brown evenly. The chicken is ready when the thickest part of the thigh reaches 165°F (74°C), and the vegetables are soft enough to pierce easily with a fork.

Move the roasted chicken to a cutting board and let it rest for 10 minutes before carving. Place the sliced chicken on a serving platter and surround it with the roasted potatoes, sweet potatoes, and onions. Spoon the rich pan juices over everything and finish with a sprinkle of freshly chopped parsley before serving.