Prepare the marinade by mixing the melted butter, lime juice, pressed garlic, paprika, salt, pepper, cumin, oregano and sugar in a bowl until everything is combined well. Refrigerate for about 30 minutes or until the butter starts to harden but still squishy to the finger.
Start by rubbing the cavity with the mixture. Using your finger, very gently pull up the skin from around the breast from both the cavity side and the neck side and with a small spoon or fingers, navigate the marinade under the skin all around the breast and to the sides under the thigh skin.
Place the half lime and onion inside the cavity and tightly tie the drumstick tips with kitchen twine to create a nice seal. Continue rubbing the marinade on the entire chicken, leaving some to use just before cooking. Cover and refrigerate for about 4-6 hours.
Ensure your oven racks are adjusted so that the roaster/ Dutch oven would sit on the lower rack. Preheat oven to 450 degrees Fahrenheit.
Lightly spray the bottom of the Dutch oven/ roaster with cooking spray and place the 1 inch potato rounds to cover the bottom. If you have a small mesh similar to a cooling rack that could be placed at the bottom to space out the chicken and the roaster, you can skip the potatoes.
Place the chicken on the potatoes and roast uncovered for 15 minutes. Reduce the heat to 350 Fahrenheit and continue cooking for 1 hour and 10 minutes, then remove cover, increase temperature to 450 Fahrenheit and roast uncovered for about 15 more minutes or until the skin is golden brown. Remember that the internal temperature of the chicken must reach 165 degrees (cook approximately 20 minutes PER POUND). Remove from oven, and let it rest for at least 10 minutes before serving. Buen provecho!