Stoneware slow cookers offer a convenient way to prepare braised meats, such as pot roast, but they're limited in size. If you need to prepare a large piece of beef for a special occasion, a countertop roaster could be a better option. They're much larger -- 18 quarts is a standard size -- and can easily fit one or more large pot roasts.
Place the cooking insert into your roaster and plug it in. Turn the thermostat to 350 degrees Fahrenheit to preheat, and cover it with its lid.
Brown your roast or roasts in a hot skillet, turning them to ensure that every surface is evenly seared. Season the beef liberally with salt and pepper, then transfer it to the preheated roaster.
Add onions, garlic, bay leaves or any other aromatic and flavoring ingredients. Pour in enough cooking liquid -- broth, water and wine are common choices, alone or in combination -- to come at least 1/3 of the way up the sides of the beef.
Cover the roaster and turn it down to 300 F. Simmer the beef until it's fork-tender, meaning you can easily pull away a piece by pushing a fork into the beef and twisting it. Depending on the size of the roast that might be in as little as 3 hours or as long as 5. If you're cooking vegetables with the roast, add dense root vegetables after the first hour of cooking and potatoes after the second hour.
Remove the beef from your roaster to a serving platter. Cover it loosely with foil, and let it rest for 10 to 15 minutes so it's easier to slice. Transfer the vegetables to a serving bowl with a slotted spoon, then strain and thicken the cooking juices to make a sauce or gravy to accompany the meat and vegetables.