Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Peel and finely dice onions and garlic. Wash all herbs and shake dry. Finely chop rosemary needles and bind thyme to form a bouquet. Wash the chicken and pat dry, spread butter all around, salt and pepper.
Heat the oil in a large frying pan. Sauté the onions over medium heat until translucent. Add garlic and rosemary and fry for another 3 minutes. Stir in lentils, stock, thyme and sage leaves. Wash lemons hot, dab dry and squeeze. Add juice and squeezed lemon halves to the lentils, bring everything to the boil. Place the chicken on top.
Bake in a hot oven without lid for about 1 1⁄4 hours until the chicken is golden brown and clear juice comes out when pierced in the thigh. If necessary, add some broth or hot water in between so that the lentils do not boil dry.
Remove the chicken and let it rest. Remove thyme and lemon halves. Finely chop the parsley leaves and stir into the lentils. Season to taste with salt and pepper.