Start by rounding up two whole chickens and pat them dry. Season the birds liberally inside and out with kosher salt and freshly ground pepper.
About an hour before you’re ready to cook, take the chickens out of the fridge to come to room temperature. Preheat your oven to 425°F.
Once the chickens are no longer chilly, dry them off with a paper towel. Stuff their cavities with lemon slices, onion, and thyme.
Using a piece of string, tie the drumsticks together and tuck the wingtips back behind the chicken.
Massage the softened butter all over the skin, ensuring an even coat. Lay the chickens breast-side up on a V-rack in a large roasting pan.
Place the chickens in the oven for 15 minutes to brown the skin.
After 15 minutes, decrease the oven temperature to 350°F. If you feel inclined, baste every 8 to 10 minutes.
In a bowl, toss the carrots, yams, and chopped onions in the remaining butter along with some salt and pepper.
A half-hour after the chicken first went in the oven, add the root vegetables to the pan.
Check the chicken for doneness about five minutes before the recommended cooking time. Use an instant-read thermometer to ensure the thighs reach 165 to 170°F.
Finally, remove the rack from the roasting pan and let the chicken rest for 20 minutes prior to carving.
