Peel onion and cut into 3-4 about 1/2" (1.25cm) rounds and place in the middle of your largest ovenproof dish.
Peel potatoes, sweet potatoes and carrots, wash celery stalks.
Chop vegetables into 1" dice and lay out evenly around onions (as seen on picture above).
Sprinkle salt and pepper over vegetables.
Wash you chicken (remove giblets if applicable) and place breast-side up on top of onion rounds.
Cut the head or tail of the lemon (only so the flesh is a little exposed) and then insert the whole lemon in the chicken's cavity.
Insert rosemary into chicken's cavity.
Cut butter into pieces and insert half of it in between chicken skin and meat and scatter the rest over here and there over the vegetables.
Sprinkle salt and pepper over chicken.
Pour 1/4 cup olive oil evenly over chicken and vegetables.
Place dish in the oven and IMMEDIATELY TURN DOWN HEAT TO 400F (200C). If you forget this step or don't do it immediately, your chicken will end up burned.
Roast for 20 minutes per pound (450g) PLUS an additional 20 minutes. In my case, for my 3 lbs chicken it ended up being 1:20 hours.