Peel onion and cut into big pieces (about thumb big).
Remove papery skin of garlic but leave the thick skin ON! (if you remove the thick skin the garlic with dry out and become very hard)
In a small bowl whisk together cumin, coriander, salt and pepper.
Whey chicken in it's wrapping or check on the label how much it weighs. (<- this is important!).
Wash and dry chicken with a paper towel. (DO NOT remove skin. If you don't like the skin or are concerned about the extra fat, remove it AFTER roasting. You should NEVER remove the skin before roasting. It serves as a protection for the flesh and without it the flesh will completely dry out.)
Place chicken breast side up in the middle of a cast iron skillet or any other small-ish ovenproof dish.
Pour olive oil over the chicken and with your hands spread it all around so it is completely covered in olive oil from all sides. Make sure it is placed breast side up again.
Now with your finger tips sprinkle the salt/spice mix over the breast, thighs, legs and wings.
Scatter onion pieces and garlic cloves around it.
Place rosemary string to the pan.
Put in 450 F hot oven and IMMEDIATELY TURN DOWN HEAT to 400 F (200°C) and roast for 20 minutes PER pound + an extra 20 minutes. The chickens I tend to get are usually 3 lbs (1.5 kg) and thus need 1:20 hours (3x20min+20min).