Take chicken out the fridge at least 30mins before you start.
Combine all of your herb butter ingredients and place to one side.
In a large tray, combine your carrots, onion and potatoes in a good drizzle of olive oil and a pinch of salt and pepper. Create space for your chicken, then lay any leftover sage/rosemary with two rounds of lemon.
Pre heat oven to 220c/425f.
Take your chicken and flip over so it is breast side down. Using a pair of kitchen shears cut through the ribs directly either side of the back bone. You can either start at the neck or the butt, just go nice and close to the back bone so you don't remove too much of the chicken. Completely remove backbone (you can use this another time for stock) and pull open up the chicken.
Open up the rib cage and slice along the sternum. This will help the breast bone pop out (not crucial, but does help. See video if you're not sure where this is). Season the inside with a pinch of salt and pepper.
Flip the chicken over and push down on the breasts to fully flatten (you should hear it crack a couple of time). If you haven't already, tuck the wings underneath (not on top of breasts). Pat dry with a paper towel (important for crispy skin) then use a spoon to carefully loosen the skin from the meat. Spoon in 2/3 of your butter then spread the rest on top. Try and spread the mixture out as evenly as possible, whilst the butter will melt into the chicken and disperse, the herbs will stay put. Cover as much chicken as possible!
Finish with an extra pinch of salt and pepper, then place on top of the herbs/lemon on the tray. It's important to make sure your chicken is as level as possible so it cooks completely evenly. Use some extra veg to prop up parts if it is not even.
Place in the oven for 45mins undisturbed, or until the internal temperature is 75c/165f with the juices running clear when pierced at the join between the drumstick and the body. Then let it rest for at least 10-15 minutes. Letting it rest will allow the meat to retain the juices, if you cut straight in you'll lose that juiciness. Do not cover, it will trap the steam and make the skin soggy.