Preheat oven to 300℉.
Quarter the onion and place spread out in empty, dry roaster pan.
Place chuck roast over top of the onion pieces.
Sprinkle the onion soup mix over top of the roast.
Fill pan with water, coming up the sides of roast about 70%. Do not cover over the dry mix! It should be left undisturbed on top of the roast.
Cover the roaster pan (with a lid or foil) and place in preheated oven. Cook for 3-4 hours.
In the meantime, prep the carrots and potatoes.
About an hour prior to dinnertime, add the carrots and potatoes in to the water surrounding the roast. Cover. Place back into oven and increase temperature to 375℉.
When finished, the carrots and potatoes should be easily pierced with a fork and the roast should be falling apart tender.
Salt and pepper, to taste, and serve!