With the rack in the middle position, preheat the oven to 170 °C (325 °F).
With the tip of a knife, make incisions in the roast and insert the garlic slices.
In a large pan, preferably enameled cast iron, brown the roast in the butter. Season with salt and pepper. Set aside on a plate.
In the same pan, brown the onions with the mustard. Season with salt and pepper. Add the broth and return the roast to the pan. Cover and bake for about 3 hours and 30 minutes or until the meat falls easily off the bone. Keep the roast warm on a plate, covered with aluminum foil.
In the same pan, bring the broth and potatoes to a boil. Cover and simmer for about 30 minutes, stirring frequently, until the potatoes are tender. Add water, if needed.
Serve the roast, cut into pieces, with the potatoes.