Combine the rosemary, garlic, lemon juice, oil, salt and pepper in a bowl. Coat the chicken well with the rosemary-garlic mixture.
Heat an oven-safe non-stick pan over medium heat. Brown the chicken on both sides. Add the mushrooms and potatoes to the pan.
Transfer the pan to the oven and roast, uncovered, until the chicken is cooked to an internal temperature of 160°F using a thermometer. Remove the chicken and mushrooms and set on a cutting board to rest. Check the potatoes with a fork to see if they are tender enough, otherwise place them back into the oven to finish cooking. Once done, remove from the pan.
Pan Jus
Using the same pan, place on stove-top over medium heat. Melt the butter in the pan and mix in the flour. Cook for 1 minute while stirring.
Add in the chicken broth and scrape the bottom of the pan with a wooden spoon to incorporate all the flavors from the roast chicken, simmer until the sauce thickens (the sauce consistency should coat the back of a spoon). Taste and adjust seasoning with salt and pepper if needed.
Spoon the sauce over the chicken; serve with potatoes, mushrooms, and bread.