Season roast on all sides with salt and pepper; coat with flour (reserve leftover flour for gravy).

Melt shortening in a pressure cooker over medium-high heat. Add roast; brown on all sides. Sprinkle onion soup mix over roast. Pour in at least 2 cups water (add 1 to 2 cups more if roast is a tough cut or to reach the minimum amount recommended by the pressure cooker's manufacturer).

Seal the lid; bring the cooker to full pressure over high heat. Reduce the heat to low, maintaining full pressure; cook 1 1/2 to 2 hours.

Release the pressure; test for doneness (the meat should be fork-tender). Add potatoes and onions, making sure vegetables are submerged in cooking liquid. (You may need to rearrange roast and place on top of vegetables or add more water.) Seal the lid and return the pressure cooker to full pressure.

Reduce heat to low, maintaining full pressure; cook for 10 minutes. Release the pressure; transfer roast and vegetables to a serving platter.

To make gravy, bring cooking liquid to a boil. Whisk reserved flour into 1/2 cup cold water; whisk into boiling cooking liquid. Cook until thickened, stirring occasionally, about 3 minutes. Serve gravy with pot roast and vegetables.