Heat some olive oil in a dutch oven. When the oil is hot, season the chicken with salt and black pepper on all sides then place it onto the oil, breast side down. Once the chicken is ready, the skin will be released from the bottom of the pot, no need to scrape it, have patience and let it cook, turn it over and brown the other side.
While the chicken is browning, whisk together minced garlic, milk, Dijon mustard, whole grain Dijon mustard and mustard powder. Add low-sodium chicken stock and mix.
Once the chicken is brown, transfer it onto a plate. Drain the oil then return the chicken to the same pot. Pour the milk mixture into the pot. Add marjoram, fennel seeds and lemon zest. Add the mushrooms. Cook the chicken in an oven, uncovered, at 200 C for 30 minutes. If you are using the stove top, cover the pot on a simmer.
Add the shallots and potatoes into the pot and return to the oven. Cook, uncovered, at the same temperature for another 30 minutes, or until the potatoes are tender. IF using the stove top, cover the pot. Serve, spoon the sauce over the potatoes.