Add the cooking oil to a lidded, large, heavy pot or 5 qt dutch oven and heat it almost to the smoking point.
Roll the roast in the seasoned flour, coating all sides.
Fry the pot roast in the pot, turning occasionally until brown on all sides. On the last side, put in the chopped onion and saute alongside the roast. When the color on the onions start to turn, proceed to the next step.
Add the beef stock, peppercorns, cloves, bay leaf, garlic powder and salt. Make sure the liquid in the pot is at least halfway up the sides of the roast. If it isn't, add water until it is.
Cover and heat until you reach a low rolling boil and then simmer on medium low 1 1/2 hours, flipping the roast halfway through. Don't let the liquid get much below one quarter of the way up the roast. If it does, add water. Ideally, when the cooking time is complete the liquid will not be more than 1/4 of the way up the roast.
Remove the roast from the pot, leaving the liquid. As the roast rests, make the gravy.
FOR THE GRAVY: Put the cornstarch into a small measuring cup and add COLD water, as cold as you can get it, just enough to make a slurry. Water that isn't quite cold will make lumpy gravy. Stir until creamy.
Whisk the cornstarch slurry into the liquid slowly and then simmer uncovered 3-5 minutes or until the cornstarch taste goes away and the drippings thicken into gravy.