Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit.
If using 2 small chickens, cut the apples into cubes and halve the shallots. If using a regular-sized chicken, only quarter the apples and leave the shallots whole.
Generously sprinkle the insides of the chickens with salt and pepper. Place one or two halved shallots and 2-3 whole sprigs of rosemary into the cavity of the chicken's body. Smear the chicken with butter all over. Generously sprinkle the outside of the chicken with salt and pepper as well. Chop the remaining rosemary and sprinkle it over the chicken.
Place the rest of the shallots and the apples into a Dutch oven or roasting pan. Add the olive oil, some salt, and pepper and mix well with your hands, so that all the fruit and vegetables are coated in oil.
Make space for the chickens in the pan and settle them there, breast side up. Roast in the oven until the inner temperature reaches 73 degrees Celsius/ 165 degrees Fahrenheit.
If you have a kitchen thermometer, check the temperature by inserting the thermometer into the thickest part of the thigh. Or insert a skewer into the thickest part of the thigh, the juices should run completely clear. If they are still bloody or pink, continue roasting the chicken and check again from time to time.
Let the chicken rest for about 15 minutes before carving.