Place the potatoes in a large saucepan and cover with cold water. Bring to a boil over medium-high heat; reduce heat and simmer until the potatoes are very tender, 20 to 25 minutes. Drain the potatoes in a colander. Return the potatoes to the pot and cook over low heat for 30 seconds or so to evaporate any water that may be remaining on the potatoes.
With a mixer, beat the potatoes until they are smooth. Add the two containers of Boursin cheese and continue beating until thoroughly incorporated. Thin the potatoes by beating in the milk or half-and-half. Season the potatoes with pepper to taste.
Transfer the mashed potatoes to an ovenproof casserole and cover.
You may rewarm the potatoes either by baking in a 350 degree F. oven or by microwaving until piping hot. Serve at once.