Preheat the oven to 375º Fahrenheit.
Pat the chicken dry with paper towels and place it in a large baking dish. Drying the skin helps it turn golden and crisp in the oven.
In a small bowl, mix together the olive oil, lemon juice and zest, Greek seasoning, salt, and pepper. Rub this mixture all over the chicken, making sure to get under the skin and inside the cavity for the best flavor.
Cut the zested lemon in half and place both halves inside the chicken cavity. Use kitchen twine to tie the legs together to help the chicken hold its shape and cook evenly.
In a separate bowl, toss the red baby potatoes and chopped red onion with olive oil and Greek seasoning. Once coated, scatter them around the chicken in the baking dish.
Roast the chicken and potatoes uncovered for 1 hour and 45 minutes. Halfway through the cooking time, gently toss the potatoes for even browning and spoon some of the pan juices over the chicken to help it stay moist.
About 15 minutes before the end of the cook time, baste the chicken again with the juices to deepen the color and flavor of the skin.
Check that the thickest part of the thigh reaches 165º Fahrenheit. Once it does, remove the pan from the oven and let the chicken rest for 10 minutes before carving.