Faster cooking times, more uniform cooking and quicker cleanup make cooking in an oven bag easier than cooking in a roasting pan, but the cooking process is the same for both. Oven bags enclose the moisture released from meat during cooking to create a contained moist-heat environment, and the hot air circulating through the oven acts as a layer of insulation that keeps the temperature inside the bag at a constant temperature. You have to flour the bag, though, to congeal the fat during cooking and prevent it from bursting.
Heat the oven to 325 degrees Fahrenheit or warmup the slow cooker. Add 1 tablespoon of flour to the bag and shake to coat.
Place the oven bag in a shallow dish or roasting pan. Season the roast to taste and slide it in the oven bag.
Tie the bag closed with the supplied ties and cut five or six 1/2-inch slits in the top.
Cook the roast until it reaches the desired tenderness, or about 1 1/2 hours per pound.
If using a slow cooker, pour about 1/4 cup of water in the crock before placing the roast in it.
Remove the roasting pan from the oven. Let the roast sit for 5 to 10 minutes before opening the bag.
To remove the roast from a slow cooker, slit the bag open and transfer it to a plate using tongs.